I would also be happy with those restrictions on a traditional PC-class computing device (laptop or desktop). Would I personally buy one? Probably not, but I'd feel a whole hell of a lot better if my non-techie wife or mother or brother were using one and they were no more susceptible to some kind of exploit on their PC device than they were on their phone
I could see Microsoft saying "we're only allowing apps installed through our 'store', for safety/security reasons, unless you opt out (gated by some scary warning that doing so is unsafe).
Even if they never charged a fee for running the store, I bet this would raise a lot of eyebrows.
That's the whole thing--there should be choice