Unlike "Dancing With the Stars," "So You Think You Can Dance" takes *any* dancer and exposes them to other styles. There are several professional dancers on it:
Blake -- the rude guy with the little dog
Snow -- she even said tonight "I am a professional ballroom dancer"
Maritza -- the salsa dancer from Chicago who chose dance over her marriage; she owns and runs her own studio in Chicago
Nick -- I believe teaches at dance camps
There were probably a lot more than that too. They just made a big deal about Blake to stir up "drama" on the show.
There were no rules against pros entering "So You Think You Can Dance."