Part 1 of 2
Bizarre.  The thing about Matthew was that he acknowledged that he was a sinner, and one must assume that Jesus didn't say to him, "You know, Matt, since you make a better living when you take more from the people than they owe, it's not a sin for you to do that.  Let your conscience decide what works for you."

The Pharisees' didn't realize that granting mercy to sinners was what Jesus was preaching - a new concept to them - so their "sin" was that they were unaware of their sinfulness.  The issue was not that the Pharisees were following the law; the issue was that the Pharisees didn't understand that Jesus was teaching them how to execute the law through mercy, not by shunning or retribution.

Both scenarios here show the same thing:  the law is important (and both Matthew and the Pharisees and Jesus were in agreement with that Matthew was sinner), but more important is to understand how to execute the law, e.g., mercy not punishment for sin.
(see Part 2)