I guess my first question would be why? Is it worth the risk? Save a few bucks on the chance they could be safe?
Snakes in the wild eat wild rodents, you are right. Snakes in the wild also often have mites and parasites both, and a significantly shorter life-span than those in captivity. (from being hunted etc also of course)
You could feed your snakes wild mice. But why take the risk? If our captive bred mice have issues, we have someone to trace it back to. If they are dirty or sickly, we know the source for which to complain / avoid.

Your barn cats are not snakes, so that comparison is irrelevant. That would be like saying well my kids eat spaghetti and it seems alright for them so my snake should be fine too. It's just silly comparing completely different
animals. Some things effect species differently.

I'm not saying your field mice are disease ridden, or are going to definitely kill your snakes or anything. I just don't see the point in risking it whatsoever.