They're not allowed to accept at my local counter, DirtyPlum got the reasons spot on it's for honesty, loss prevention, security purposes etc here. I work in the same department store and they spell it out in massive letters that none of us can accept any gifts whether it be money or chocolate or anything like that. It's a shame really because some MAs have been so lovely and helpful to me that I'd like to give something in return to boost there day.
I like the idea of writing though - I'm sure if Head Office received a letter complimenting MAs at a certain location there would be rewards involved for them. I keep meaning to write about my local Lush as well, they are lovely in there.