Privacy mainly. And as a software developer, such app stores are insulting to us because it is used to extort money from us for the "privilege" of developing software for their platform, when they should be the one appreciative of the fact that every new software from us increases the value of their platform. As a user too, I find this very unethical of them as it unnecessarily increases the cost of buying softwares (why should I pay Apple or Microsoft or Google an extra 20 to 50% for buying a software made by you?). I have no qualms in saying that we software developers are fools for letting them take advantage of us and our clients / users.