Not sure this was regulating someone. But on a recent Southwest flight, the 3s were loading and it appeared that no one was going to sit in my middle seat. So a young woman boards and is told there's no room for her large bag. Being the gent that I try to be, I offered to move my bag from the overhead, shuffle things around a bit and put my briefcase under the middle seat. I wouldn't fit under the aisle seat.

Sure enough, grunge-guy boards in tattered shirt and jeans. Asks if he can sit in the middle seat. I explain that he's welcome to, but there's a briefcase under the seat that shouldn't bother him. Guy tells me to move it. I tell him there's no place to put it since I'd given up the overhead space. Woman I gave the space to offers to let the guy put his bag under her seat. Guy walks away muttering curses at me.

Just another no good deed go unpunished when flying story.