I disagree with the general notion that making an app that is compatible with an operating system (i.e, that calls its APIs) is a "use" of the OS maker's IP, at least on a legal sense (which is all that matters in this context).

For furher clarification, please see this fellow HNer's comment and its response by user kmeisthax: https://news.ycombinator.com/item?id=39564383