Don't know about rude, but I've seen plenty of useless ones; I am not blind or dyslexic, I can read the details from the label myself, I don't need someone to read them out to me, I want someone who can tell me something that's not on the label.
The OP has a good point though, there's no reason why a shop assistant shouldn't be in a wheelchair, but I don't recall ever seeing one. In fact I would have though it would quite a convenient job for someone in a wheelchair, with what should be a wheelchair friendly environment.
EDIT : Wife has just pointed out that a lot of shops expect assistants to work in the stock rooms too, which might not be such a wheelchair friendly job (high shelves, etc). However, she says she has seen wheelchair users working in banks, which don't have stockrooms.