Well, if you look at the numbers, the VAST majority of the PC/360 titles are almost 360 exclusives, concerning the sales.
I mean, look at CoDMW2, the console numbers are destroying the PC sales (due to piracy of course).
So, for the company, either you play or not a pirated copy on your PC (I don't speak about YOU in particular, it's a general argument) doesn't make much of a difference.
These PC/360 titles, almost act as if they are 360 exclusives in the sales department.