I've had horror stories of bettas and shrimp. I had a sorority kill 3 BIG pregnant ghost shrimp JUST to get the eggs.
Onto the original topic of snails and bettas. It's been stated into overkill that bettas are individuals, some bettas won't tolerate ANY sort of tank mate, others, you could probably get away with throwing anything into their tank, and they won't care one bit. On the other hand, snails are individuals too, and it's the ones that learn to protect themselves that tend to live longer. Some snails just don't develop those particular survival instincts. I have a trapdoor snail of some sort that I collected from a lake last summer, and this is honestly my most successful snail out of all of them!
About comparing what snails go through in the wild, compared to tanks. I feel it's appropriate to make that comparison, because the inhabitants of our underwater worlds don't really know to act any differently than they would in the wild. Bettas eat small invertebrates, shrimp, snails, bugs. And people culture these things specifically for their bettas to consume. Since they're trying to replicate nature as much as possible for their pets, is this wrong too? Knowing as much as we can about the wild relatives of the animals we share our homes with, helps us to better care for them.
In closing, just like some bettas don't do well with other livestock, some snails don't do well with other stock either. And I would consider an invertebrate (shrimp and snails) tank for your snail, and fish only setups for your bettas rather than returning them (because they deserve good homes too). I hope your snails recover from the social experiment that the bettas didn't comply with. They must not have gotten the memo that they're supposed to be nice ;)