Want to try something fun? Install any browser in iOS and do a test. Then do the same test with Safari. Now ponder why they are exactly the same no matter which browser you install.
If you're referencing Apple's mandate that WebKit be used as the engine for all iOS web rendering, then yes, that's a poor policy ostensibly made in the name of security. Not sure if that's what your point is, though.
Can you imagine how much legal trouble microsoft would be in if they blocked alternative browsers in windows? Simply providing a single default was too much.