There is (perhaps not) a surprising number of folks who buy radio who don't subscribe. They go to All Access or Radio Info (or any other online source) to get 12+ numbers to see who's on the radar. I know, I've called on these buyers and some are dealing with a bunch of bucks. So when a market is embargoed, the non-subscribing buyers see only the rankers that the subscribing stations choose to show them.
Subscriber only quarterly releases are particularly troublesome. It's like showing a group photo with several folks airbrushed out. Who reads the fine print? I've received e-mails from folks who say something like "hey, good showing in the book," but those results don't see the light of day.
Arbitron owns the numbers and what they do with them is their business. Membership has its price...and it's benefits.