Xbox and Google have apparently provided conflicting opinions on whether Xbox is allowed to sell and release games from its Android app. In October, a US judge issued a permanent injunction ordering Google to open its Android market to competitors. The ruling, which was supposed to take effect this month, means that Google will not
It’s more complicated. Apple allows it, but apply a lot of fees so that it is really expensive (and probably not viable). If I remember correctly, you can propose your own store but you have to pay a fee for every download on the store (even for free apps).