Apple likes to control their iPhone OS platform. Why would they let people code in VS and lose control? The Mac is an open platform however, so I guess Ballmer could let people develop for the Mac until their faces turn green. I don't see how that would make any money for Microsoft nor Apple. This doesn't have anything to do with the actual quality of VS as a development tool on the Windows side.
Oh please, prior to Microsoft eating the OS industry, Apple and MS were pretty good teams. Offfice was first on the Mac and I think Microsoft helped develop the programming code for System 1.0. I could be wrong though on the programming - still searching. The only reason Apple hates MS' guts is for competition reasons, and the whole GUI issue.
I mean, Apple let's you run Windows on a Mac now!