A company doesn't need a majority of the market to be prosecuted for anti-competitive behavior and likewise having a majority of the market does not mean your every move is illegally anti-competitive.
If prosecuted under the Sherman Anti-Trust Act, existence of a monopoly is a precondition. Certainly there are non-Sherman charges that could be brought, but I'm not aware of anything that would be applicable in this instance?
To your first point, assuming Sherman is used, I believe US v Alcoa [1] is still the controlling case law. And you'd have a very hard time convincing a court that "smartphones made by Apple" constitute a product category over which it could have a 75%+ market share.
Or are there other laws under which action could be brought in this situation?
