Since they're just ads for other things by Rovio (which, if I were a big Angry Birds fan, I'd personally be happy to see: I usually like to find out other stuff made by those who make stuff I like a lot), I wonder if they could've mitigated the backlash some by making it easy to choose not to see them again.
Anyway: convention is that paid apps don't have ads. That's tough to overcome, and everybody gets more pissed when the big guy seems like they're taking advantage of their position.
The title is inaccurate here: they didn't add ads to the paid app, they added recommendations for other games made by the same company. The original title, with "ads" in quotes, is more appropriate.
I am looking at my paid version of Angry Birds. It contains ads for Angry Birds plush toys, an original CD, and St. Patrick's Day cards, just to name a few items.
Anyway: convention is that paid apps don't have ads. That's tough to overcome, and everybody gets more pissed when the big guy seems like they're taking advantage of their position.