I think I addressed this in the comment. To clear, I'm not a fan of Apple's policy on this and think they should allow other engines. Having said that, I think for 99% of users, the underling web rendering engine is an implementation detail they aren't concerned with.
The funny thing is that this policy is one of the few things keeping WebKit alive, if not Chromium would become the only usable browser engine (and Firefox but people would probably stop supporting it if it was the only alternative browser)