From FC Delco's FAQ
Which teams participate in 10-month programming and do not allow high school participation?
The entire Academy program does not participate in high school programming. For a complete list of clubs and more information on the 10-month season, click click here.
Seems as though they Academies implemented this policy in 2012. So it's a recent thing. they say it won't effect HS soccer, because they are only siphoning off the top 1 or 2%.