When you think that Microsoft was accused of abusing its monopoly when they shipped IE with Windows. You cannot even install another true browser on iOS, just some sort of chrome around safari/Webkit.
It would require a new set of laws to have anti-trust apply to ecosystems rather than only industrial/sector/field monopolies. At the moment Apple is free to do as they please.
There is competition in software, btw. You can buy an Android or Windows phone and get a completely different web browser in them.
Apple also doesn't like unsigned code, and JIT (or self modifying code, or whatever falls into that category) is inherently unsigned code.
I think that effect could be fairly large if Apple would allow applications to contain their own web engine, not because it would mean a second engine, but because it easily could mean a third, fourth, etc. engine, given that not all applications get updates at the same time. Imagine app #1 shipping with Firefox's webview version X, app #2 Firefox version X with bug fix Y, app #3 with Firefox version Y, etc.
The restriction is plainly a business decision, not a technical one.
Using something else if is a viable option for most people that currently use Apple products, where it wasn't for many in the case of Windows back then.
Until then, Apple is free to control 100% what comes into their platform.
The same way any other vendor is free to do (e.g. games consoles), when not covered by monopoly laws.
When they reach a size that falls into what the law considers a monopoly, then the game changes, as they have increased responsibilities towards the market.
A handful of large players, each with less than 50% market share, can also distort the market and engage in anti-competitive tactics.