I got this information form the Species mixing disasters page of Caudata culture. Also the common sense that snail shells have the possibility to impact axolotls since they are indigestible. Am I wrong?
Caudata Culture Articles - Species Mixing Disasters
I don't think that wild axolotls avoided snails, but also compare the # of snails (or clams in this case) per square foot of ground in a tank to the # of snails that exists per square foot in Lake Xochimilco. Statistically, an axolotl is much less likely to encounter enough snails (or clams) in the wild that it would become impacted, due to the likely fact that snail population density would be lower in the lake, and the fact that there are an infinite number of paths to travel through the lake. It is possible that an axolotl may never encounter more than one or two snails in it's lifetime in lake Xochimilco (not guaranteed, but possible). In a small, closed system, like an aquarium, there is no possible way that an axolotl won't notice each and every shell dweller in it's tank.
If they're small enough to eat, too big to pass, and enough of them, the axolotl will become impacted. If they're too big to eat, they can injure the axolotl by trapping them in their shells (if they are clams), or their operculum (if they are snails). It's too much of a lose/lose situation to risk it.