They do sell frozen rodents of various sizes. It wouldn't be very ethical of them to sell pet snakes, and then not sell SOME kind of food for it to eat.
That being said...I know that as a general rule, they are extremely resistant to selling live animals as feeders. And I imagine those employees will say just about anything to deter folks from buying the critters as feeders.....sometimes it may be company policy they state...it might be local management that tells them what to say...or it could be their own ignorance or their own willingness to make up whatever sounds good to scare off the evil snake owners. Who knows?
Primary thing to realize is that you can't trust 95% of what comes out of a pet store employees mouth when it comes to reptile care...and since you never know which 5% is true, you'd best just not believe any of it.
Buying live rodents from the big box stores is THE most expensive way to feed your snake, btw...there are less expensive alternatives.