You DO see a lot of 3 legged greys but it's simply because they're spindly-legged, uncoordinated idiots, not because someone let them off lead. My hound, when she was about 3 or 4 (shes 6 now) ran STRAIGHT into some sharp metal in my backyard. That was a lovely expensive 8pm vet visit. And she didn't want to come back from the anaesthetic (very common with the breed due to their extremely low body fat) so it turned into a really frightening experience.
It's not just big greyhounds either, an Italian can break its own legs getting down off the bed (so if you have an Iggy and it's allowed on the furniture, NEVER let it jump down) and I had a whippet mix who we're almost certain had broken her neck at some point in her life. Rescued so very fuzzy on her history before we got her, but she showed quite a few signs of an old spinal injury.
But yeah, relating to the on-lead thing, I don't see the sense in legislating, but it's definitely smarter to keep them on the lead. Most responsible sighthound owners will say the same.
MAKORA THOROUGHBRED SPORTHORSES