It's not in Activisions best interests either, gimping PS3 versions of games will ultimately effect sales, which directly effects Activision's revenue/profit not Sony's. That is a double edged sword.
The multiplatform games of Activision/EA are not going to make or break a console. Exclusives make or break a console, that's why when Konami/S-E complained about PS3 price Sony said nothing and lowered the price. Activision lose a lot of their own power because they make all their games multiplatform.







