A lot of different cosmetic companies offer professional discounts (not just MAC) PRO members do pay to have this advantage. I also agree with makeupgrrl that an artist who makes their living doing makeup probably needs to buy in bulk more than just a normal consumer. (Not to mention they need shades that look good on a variety of people, rather than just one person)
Also, just because something sells out during the PRO early release doesn't mean it's gone. It's my understanding that they set aside a set number of products for PRO and save the rest for the sitewide launch, (someone please correct me if this is not the case) not to mention all the product that is shipped to MAC stores and department store counters. I know it can be frustrating, but don't give up just because it sold out on PRO. Keep stalking, Colormefab! You'll get your chance!