It's about the question of engaging in anti-competitive practices and Apple is guilty of those despicable practices.
However, I must say that I am not totally sold on all the examples cited in the ancestor comment esp. that developers having to work exclusively on Mac machines to build for iOS devices because that's on the business side of the equation and mostly motivated by financial gains.
On the other hand, as a consumer I'm severely affected negatively by the restrictions Apple imposing on the browser engines available on their mobile devices and the monopoly Safari WebKit enjoys there and I'm really frustrated by that and by the fact that no one yet went after them for this but when you take into consideration that a lot of the big guns in the system are investing in this evil corporation, things become very obvious to you.