Earlier quoted context omitted.
They have no other choice; Apple won't accept apps with custom web rendering engines.
How can Apple get away with that? I am genuine interested! Sounds pretty much like the same thing Microsoft did with Internet Explore back in the day, except they didn't force you to use their rendering engine, but shipped the OS with their browser as default?
Restricting what developers can and cannot do on a platform allows Apple to give more guarantees and reliability in terms of performance and battery usage, as well as security and stability. Those are the primary reasons behind Apple's restrictions on the iOS platform.