from what I can tell, DCA itself runs three shows a year....Scranton, Prelims and Finals. I suppose everything else is left to the organizations/people that sponsor those shows.
Cabs run shows in Wildwood and Clifton that have historically had non-DCA corps performing...numerous alumni corps in addition to the Cabs Alumni and <gasp> DCI corps )(Raiders, Jersey Surf, to name two). I could be mistaken, but I believe that several years ago when Bucs home show was earlier in the year, there would be some DCI corps performing.
Can't paint DCA with such a broad brush, IMO.