I heard it said that if d20pfsrd didn't use ads then they wouldn't have those legal issues, could use the proper names, and would likely draw in more users.
I aso imagine it is also why many people flock to other sites and why they didn't get a contract with Paizo like AoN did.
I for one don't pay for ad free interfaces. I pay for the quality services themselves. That's why I'm a proud patron of AoN. Anyone using d20pfsrd's business model will never see a dime from me. It's essentially a lesser form of blackmail.