In real life, that's very difficult for most people to say.

Yep. The LW already knew that parking in the handicap spot was wrong. He asked for validation, which Amy rightly gave, but the real issue is that the MIL is demanding, perhaps controlling and/or bullying, and the LW doesn't feel comfortable standing up to her. Even without the issue of the handicap permit, it's kind of weird for a passenger who doesn't have accessibility needs to insist on a particular parking spot.