(1) No one currently cares. Pools, for example, can rob their miners undetectably... they _could_ do things to prove that they were not cheating, but none do. Miners could use P2Pool which can't rob miners, but most do not. I imagine the story will be the same for cloud mining services, and they'll be able to use even more arguments about reputation and such to claim their honesty.
(2) The standard Mike Hearn argument: Part of the reason that Bitcoin is viable is that its very comprehensible (at least in broad strokes, if not for the subtle details) to basically any technically minded person, invoking some kind of zero knoweldge thing to _enable_ a specific kind of cheating is far more complex than anything currently in Bitcoin, even if it is a neat idea.
Both of these are reasonable points, at least for the time being. But think of this as a long term idea. It's hard to project how people will behave in the future, as Bitcoin becomes more widespread and familiar. I'm pointing out a potential concern, and if it eventually becomes the case that hosted mining catches on, and clients demand proof-of-honesty from these services, then here's a solution ready to go.