I see this happen way to much with my friend's chocolate Labrador.
He has major people issues, but loves going on walks. I've managed to befriend him, but it freaks him out to no end when people go right up to him and start petting him.
So far, I've only ever had one person want to pet Mikey, and she was polite enough to ask if her kids could pet him. The kids were wonderful too, Mikey adored it :3
There needs to be more respectful people out there, not ones who think they can touch or grab whatever they want. :C
I'd probably bite them before Mikey, but still!