Lots of people dont' have a google checkout account. Not very many countries are used on the buyer side, and even few on the seller side. Additionally, its shitty as hell to setup on the phone if you've never heard of it.
I'm not familiar with the billing issues mentioned in the article, but it seems to me that Rovio is just trying to leverage themselves into an advantageous position in the market. There is big money in being a payment system provider as has been shown by Apple and Valve.
It could be very easy for them to force their way in the door too since they will be using the carriers to accept payment. This could give them huge support from cell carriers since they will be recieving a cut as opposed to the iTunes Store where Apple keeps 100% of the profits.
The game quickly turns into a pachinko machine as the levels get more difficult. I think it takes advantage of pachinko gambling psychology for a lot of its appeal.
