Likely it was AdBlockPlus that acquired them to expand their installed base by 40 million users.
ABP had earlier 'adopted' the 'unobtrusive' ads model (they always controlled which ads you saw -- the acceptable ads), which translated probably means "we'll show you the ads by people that pay us to show them." Problem here was that users could not really control that decision, and so millions (me included) switched to AB, which was a code fork from the FireFox extension AB gang that was refactored into the Chrome extension many use today.
If you look at the exception rules for AB and ABP - the both use the same list now -
anything marked with @@|| which means 'show anything from this page', the likely companies paying to play quickly emerge: Google (search, maps, adSense, soubleclick.net), Amazon, About.com, eBay, etc.
This should surprise no one, ever. Ads are huge revenue sources and if you can't control the blocking vendor, then you pay to play.
On AB, at least, you can opt out of this. And both programs can block all ads, though the CPU and page load time cost usually isn't worth it.
Users may cry foul and switch to uBlock Origin (I have for now) but in reality this is capitalism pure and simple -- someone adds a 'feature' that a customer is willing to pay for -- it is no different than an online bookstore or music store taking a cut of the revenue from producers that want their items sold and/or featured by that store. Or put in a bricks/mortar way, the ad producers are wholesaling their product to the ad blocker middleman to be merchandised by the ad-supported website retailer. How is that hurting the user of the browser extension? We still can opt-out, create our own blocking and exception lists, etc.
So in answer to Victoria's original question, I would say that if AdBlock did not inform users of the unobtrusive ads 'feature' or had a secret backdoor list of paid-in advertisers that the user had no control over, then many would consider that a sell-out, i.e. making of false promises to its user base. As it is, they are being aboveboard (so far) and the code is open source so anyone who cares can review the ad exception processing rules code and/or clone the repository and make their own ad blocking extension.
Finally, let us all be careful about the 'sell-out' characterization -- most of us live and work in a part of the economy where M&A or IPO is the end game, lest we offend our finance masters by not executing a proper exit strategy! :-)