I know it's a problem. I would be bothered if someone else's cats roamed into my yard. I totally understand where other people are coming from; cats digging up their gardens.
That's why one of my concerns is that people like us that are trying to curb the problem of cats are the ones that may end up getting punished. I always say two wrongs don't make a right. Let's reward people who do what they're supposed to do.
It's sometimes discouraging when you're not sure if you're making a difference.