My Betta's thought that snail antennae were food. Since Betta's love live foods (bloodworms, blackworms, etc.), I would think snail antennae would be very much like a Betta delicacy. If the snails learns (and it will) to keep the antennae rolled up under the shell, it can keep the fish from nipping them off. However, it won't keep most Betta's from pestering the snail(s) to see if the snail will slip up and leave one outside the shell just enough for "a little nip". This has been my experience, anyway. I never liked keeping my snails in with my Betta's. I don't think it could be a very pleasant experience for the snail and could very well contribute to their untimely death. IMHO
"If you always do what you've always done, you'll always get what you've always got..."