A bit about the d20 System License
G'day!
By the terms of the d20 System License, 5% of any product displaying the d20 System trademark must be Open Gaming Content.
If I understand it correctly, much of the AEG books are description rather than rules content; however if they want to put a d20 trademark on them, they've got to have that 5%.
So they put in Prestige Classes and Feats to meet that 5%.
I mean, they might want to do those PrC and Feats anyway - they probably do; however, they do need 5% OGC before they use the trademark.
Cheers!