Whether it's rude is a matter of local custom.
Whether you're offended by it depends on your flexibility and sensitivity
Often, we cyclists can become a little over sensitive, after having to defend ourselves from car doors, pedestrians, cars, trucks, laws that don't take us into account, police who have a ticket quota, and even other cyclists.
Relax.
The person who left it there is just trying to save themselves the trouble of carrying it every day.
And yes, you can still park your bike there.
Of course they may think you're rude then, so I'd only park there if it was the last position available.
bicycles.stackexchange.com/users/8273/andy256
I'd pick a different spot in the rack if possible.
But if I was in a hurry or there wasn't another suitable place I'd use it (e.g. my bike doesn't fit on the bottom of the double stacker at the station; some places have decent racks and others that don't allow proper locking for all day use).
I would try to make sure it's easy to get the lock off, perhaps by sliding it to the end before parking.
Blocking out a space is properly selfish and possible in some racks (e.g. a lock right across a slot-in rack).
I wouldn't show any consideration to anyone who does that.
Luckily it's rare.
In many places if you want to store a lock, the done thing is to lock it to something like a bike shed pillar or the outer side of the outer rack, so it's easily accessed and not in anyone's way.
My views on this are perhaps quite harsh, as I park at a couple of stations where some people park all day and others all night.
Overlap is inevitable.
Offices are rather different in that they tend to be occupied for the same part of the day.
bicycles.stackexchange.com/users/7309/Chris H
This will depend on MANY things, including but not limited to: local custom office politics special concerns (I can't think of any, but more in this later) utilization of the rack your feelings on the matter type of lock Local custom is important.
In some areas it's horridly rude; in others it's common.
Does the parking complex for cars have assigned spots?
If so then the lock on the rack is probably just an extension of that.
Office politics play a big role.
Does the lock belong to the owner of the building and buyer of the rack?
Because if it does, you're out of luck.
It doesn't matter if it's rude or not - you're not going to win this fight.
Or does it belong to an employee who has made a stink in the past and the proposed solution to some problem was to leave the lock?
I can't really think of any special concerns that would affect this, but I do have a similar example from when I used to leave the lock on the lockers at the company gym.
Essentially, I was told it was rude to leave the lock on the lockers at the company gym.
But this was an arrangement I made with management because I would frequently work half a shift, work out, shower, return to work the other half of the shift, and leaving smelly work out clothes (and I can be smelly) in the locker room was better then stinking up the office.
And, because my time around the campus was extensive, in the morning it was better to deposit the clothes in the locker then to have the gym bag around my desk.
If the rack is under utilized, and you know it's going to stay that way, it's a much smaller issue.
For example, a 25-bike rack with an office size of 20 people and only 5 of them bike.
Then it's not rude at all.
This is tricky.
But make sure you have a legitimate reason to be upset.
Often we bikers think of ourselves as "last on the list".
We see it as a treat to have a bike lane that's clean and free of debris.
Make sure you're looking at this issue objectively.
I would add that the type of lock is important.
Our local bike shop has around ten types of lock, ranging from a simple cable with built in lock weighing around a pound to massive, complicated bar and cable "systems" that weigh closer to 25 pounds.
Their most popular option is a 5 pound cable and U lock combo.
Either way, I would not want to carry around some of their heavier locks.
bicycles.stackexchange.com/users/25324/coteyr
Does the lock make it harder for another person to use that space?
If so leaving the lock is rude.
If not then it is not rude.
If parking your own bike in that space makes it harder for the lock owner to get their lock off so they can use it in another space, that's really for the lock owner to have to deal with and nobody else.
Probably the small inconvenience outweighs having to carry a (possibly very heavy) lock on a daily commute.
In the bike room at my office many people leave locks on the racks, never an issue.
bicycles.stackexchange.com/users/1914/Adam Eberbach
The polite thing for the lock owner to do would be to "store" the lock in a place in the rack (eg; at the base) where it's not in anyone's way.
If it's stored in an inobtrusive place or manner,then I'm ok with it.
Otherwise, if it's in a prime spot, it is an unauthorized reservation of that spot.
I often encounter this in a crowded bike rack, and I find it quite rude.
bicycles.stackexchange.com/users/26746/little ray
We have a bicycle parking rack near the office building. It's rather large: for about 30-35 bikes.  Someone has left their lock (chain or whatever) locked around one part of the rack.  So, I've got two questions:   is this rude, or it's just me thinking this way?   can I park my bike on such a place if there are no other options?
Leaving lock on the parking rack - is this rude?
bicycles.stackexchange.com
<lock><etiquette><parking>
bicycles.stackexchange.com/questions/40684
bicycles.stackexchange.com/users/4007/k102
Leaving a bike lock on a bike rack may be considered rude in some circumstances and not in others. It mainly depends on the local custom and office politics. If the bike rack is under used, it is obviously not an issue. I, however, it is preventing other people parking their bikes, it is rude. Other riders are still free to park their bikes in that spot and if the lock owner cannot retrieve their lock, that is their problem. oof course the lock owner may just be saving themself the trouble of having to carry the lock every day. TROUBLE OF HAVING TO CARRY THE LOCK WITH THEM EVERYDAY. If there are enough free spaces or the lock is not blocking a prime space, the lock was probably just left by the rider to save carrying it to and from work every day. It depends on local attitudes and customs, but it's normally polite to lock it out of the way, and for other riders not to obstruct access to it if possible. You can avoid that space if others are available, but reserving it would be unauthorised so you can park there if you need to.
If there are enough free spaces or the lock is not blocking a prime space, the lock was probably just left by the rider to save carrying it to and from work every day. It depends on local attitudes, but it's normally polite to lock it out of the way, and for other riders not to obstruct access to it if possible. If the lock is used to reserve a space, this is normally considered rude, but otherwise it can be ignored as a minor issue.