Controlled Exotic Snake Permit (a.k.a. Nonindigenous Snake Permit)
It has to do with how prey animals are treated until death. Some people would rather sell off dead rats that were humanely and specifically killed to be feeders as opposed to selling a live rodent intentionally as food and knowing only what sort of unholy treatment awaits it. (contrary to popular belief, there's quite a few people out there who treat feeders abusively and like utter crap because they think it doesn't matter that it feels pain because it's going to die anyway - youtube is proof enough of this) And there's quite a few people out there who are against live feeding as well - including many reptile keepers. There's some people who just hate it because they're emotional softies, while many of the aforementioned reptile/exotic pet keepers see it as just an unnecessary process full of risks and unneeded suffering, also claiming the "nature" argument is a logical fallacy lacking any weight or rational basis.
And look at it from their perspective - it must really mean something to them if they go so far as to actually deny a sale to the customer. Same could be said of people who deny the sale of something like an iguana because the potential customer planned to feed it crickets and rats. If it bothers you so much, either buy their dead rodents or go to another store.