Suppose that Google is sick of Apple and their patent lawsuits and decide to kick it up a notch: They block Youtube on all i-devices. It is trivially easy to do, eg by user-agent filtering. It is also easy to circumvent, but a lot of non tech savvy users will still be affected.
Aside from that being a dick move, hurting their image, turning away customers, etc. Would that be illegal in any sense?