Rovio said that it was an "obvious choice" of GetJar over Android's Market, thanks to the "global reach and popularity as well as their unique App It! download service".
Although it's free to the Android user, the guys at Rovio won't be living off baked beans and cup-a-soup, you'll be pleased to know, as it's ad-supported. That's not a big problem to you guys, is it? Extra Angry Birds levels in exchange for a few ads? [GetJar via Mobile-Ent]