I don't know the EXACT reason since I do not work at a store or handle any discounts at all since I work at a Macy's counter. I think that it's probably that since it's a "special" collection that they want it to be a luxury collection, and only be full price. However, since they are technically standard MAC items (skew wise) the artist would have to adjust the pricing during check out.
Something like this has happened to me as a customer. I have bought bags and accessories that are supposed to be 40% off for artists, not the standard 60%, but gotten the full 60 off. That's all I can really think of so it's just if the artist knows whats going on or not I guess? I really have no idea, this is just speculation.
That said, I got a Dame trio the other day with the full employee discount. So... goodluck!