I wonder, too. I suspect that some will shoot and then claim that they didn't see the collars. Oh, well! Or, they will shoot the cats, remove their collars from the dead bodies and perhaps toss the bodies into the bushes. No collar -- okay to shoot.
Further, how do you know a cat is really feral, and not a farm cat? Don't farmers keep barn cats to control rodents?
I have a number of friends who live in the country and their cats go outside minus collars. One of my friends owns a horse and I've visited the various barns that he has been kept. The barn owners keep cats and I don't recall seeing collars. I may be wrong, though.