Some comps allow pros. Others don't. Some comps would allow beer brewed in a brew-on-premises facility. Others won't. It is always up to the competition organizer(s). Its their comp, so they set the rules. If the entrants don't like it, then they will vote with their feet.
Honestly, I see nothing wrong with restricting entries in this way, so long as those restrictions are prepublished as part of the rules.
All that said, you mention getting the "very best" brewers. Why would guys like Vinnie Cilurzo, Sam Calagione, Matt Bryndlson, or Tomme Arthur want to enter a homebrew competition? They wouldn't. I also would think that once I went pro, I'd be more interested in competing in comps with other pros.