Why is MS afraid of OEMs?

I am wondering why many think MS should worry about its relationship with the OEMs when they plan to release the surface. I see no point in MS being the least afraid of its OEMs because their OEMs don't have a choice. Windows is the only OS they could put into their products, Linux is there but many of the consumers aren't familiar with it.

Could somebody explain to me why MS should be afraid of its OEMs?