Netscape used to be a paid product* before Microsoft made IE free. What Microsoft did was make a free competitor and install it on every machine running their OS.

So what Microsoft did that was illegal was use their monopoly power to destroy an existing player. That's how they were "anti-competitive".

While Apple not allowing any competing rendering engines to be written for iOS is anti-competitive in the dictionary sense that they are preventing competition to be created, it isn't the type of behavior targeted by anti-trust law.

(IANAL, but my understanding is that if Apple had removed Spotify, Pandora and other music apps from the App Store while releasing their streaming music service, that would be an anti-trust violation.)

*Netscape was $49 in 1996 http://www.fastcompany.com/27743/nothing-netscape