People play a lot of out (usually long) serves in rec play. It can be tough to tell in singles (which is what I play).
Even other than serves....I get to watch a lot of rec and league (USTA & others) matches from waiting areas up above courts and there are tons of bad calls other than serves in rec/league tennis. I don't think most people are cheating at all. But rec/league tennis is a big source of self-esteem for some people and they see what they need to see psychologically. It may be pathetic but it is essentially subconscious, the people who do it are otherwise basically decent folks, and you'd never get people to understand/admit that's what is going on. This is just what I've observed.
I don't let it bother me. When I play people making bad calls, I just imagine myself across the net from a pubescent adolescent with that desperate need kids have to prove they are worthwhile. Just smile, be the grown-up, and don't indulge the annoyance that will likely affect your play.