Those individuals entered that particular tournament as Pro's. Sometime after they entered, they became Masters by continually scoring low in Ranked rounds, thus achieving the necessary average to become a Master. From this point forward, every tournament they enter will be as a Master.
Once you enter a tournament at a certain tier you stay at that tier in that event until it ends, you can not enter the corresponding Master tier event.
If it's an unlimited play tourney you can continue to play in it until it ends, even though you are now a Master.
Here's what makes it fair... If those newly minted Masters purchase Master level equipment and then try to play in a Pro tier event with the new stuff, they get an error and are unable to play. They may only play in a pro level tier tournaments with pro level equipment, therefore it's a level playing field for everyone. Hope that helps.