I think the problem is being sure which ones are really feral. If "no collar" is all that's needed for a cat to be shot legally, then cat-hating hunters will just remove the collar afterwards and surreptitiously dispose of it. My cats never leave the house without a collar that has both rabies vaccination tag and nametag with my phone number, and I don't think anybody's should, but I don't think people who enjoy going around shooting cats can be trusted not to tamper with them.
My cat is an indoor cat, and she still has those credentials, just in case she slips out while I'm not looking. She also has her own e-mail address on the tag, in case I'm out of town when she flies the coop. And yes, she has gotten letters from Nigerian scammers.