
Actually, it's a bit more complicated than that...I sounds to me,
your UPS takes in AC, rectifies it to DC to charge the batteries, and
then takes the battery DC and chops it to AC again, feeding your
equipment.  This approach is the easiest and cleanest way to
switchover from the mains to battery once your power kicks out since,
as you mentioned, nothing will know about what happened down the line.

Another way to do the UPS scheme is to use the mains until you
lose power, and then kick in the battery backup with it's inverter to
replace the lost power.  The problem here is the switchover time and
you've got to resync the AC in no time flat.

Unfortunately, most everything is built around the assumption that
AC is available, so the UPS guys have to provide and AC output to
be usable...ya sorta have to make it work with what there already.

Similar story with our telephone system.  It was first invented back
in the 1800's.  We're still using the same damn system (media) as they
did back then.  If I have a phone from back then, I can assure you
it'll work on today's phone system.  It costs too much to overhaul
everyone to a new system, so they make it work with what is out there.