There's a minimum size limit, Joe. These legal turtles are, for the most part, already sexually mature.
Farming mink has nothing to do with any of this. Guys trapping mink and rats in the marsh aren't trapping farm raised animals. They're trapping a viable population of a resource at a sustainable rate... For now. Turtle trapping is no different. And there aren't many people going after them anyway. I live in the epicenter of snapping turtle country of Ohio, and I can count on a few fingers how many serious turtle trappers I know. There's no shortage of turtles.
Now, is it fair that someone can trap and sell a turtle but I can't go out and catch a walleye and sell the meat? No, it isn't. And I can't shoot a deer and sell that meat either. But why can we sell the pelts of furbearers? Most of those don't even get eaten. At least turtle meat is edible. It's just because that's what trapping is all about and that's been the tradition for hundreds of years. If we were running out of snapping turtles, I'm sure things would change. Trust me there are plenty of antis that would love to see that happen. You ever notice how many snapping turtle nests get raided by coons? Damn near every one of them! Yet somehow we still have a shit load of turtles. Evidently a few always seem to evade the coons. If the coons can't eradicate snapping turtles, chances are the small number of trappers targeting them won't either.