I wouldn't call them a liar so much as they just have no idea. I imagine a lot of pet store employees are trained into believing that something good for one rodent is good for another, which isn't true.
It's like how those "rat and mouse" mixes are sold on the store shelves. Those aren't actually that great for rats. However, if you knew nothing about rats and saw that on the shelf day after day, wouldn't you assume that it was ok for both because it says it?
Companies, I think, tend to mislead more than they should.