Microsoft is throwing money at them and the Xbox 360 was the American multi-platform console of choice because of entry price and Xbox Live. Microsoft and EA have the same vision for the gaming industry, so it was only a matter of time. EA wants maximum profit so the chances of multiplats coming from them are higher than exclusives.
Microsoft and EA see eye to eye when it comes to policies and gamers, but as you saw at E3, EA will leave MS to take the beating if gamers turn on their anti-consumer politics. I also wouldnt be surprised if MS threw at least half a billion or even a billion at EA for preferential treatment.