Why is this reverting to marketshare of phones? It's secondary to the problem. It's about influence on the market of browsers, which also affects tons of other things in result, because so many services today are Web based.
(INAL so take this with the appropriate amount of salt)
Because it's illegal to use a dominant position in one market (desktop operating system in Microsft's case) to gain an unfair advantage in another market (browsers). Since Apple does not have a dominant position in any market so they're free to do as they please.
Note that the question what constitutes a market is not always easily defined. Apple obviously has a monopoly on iPhones or smartphones running iOS but I doubt you'll find a judge that sees that as a market distinct of the wider smartphone market.
Well, Apple's ability to stifle the progress of the browser market, demonstrates that their position is mobile market (which is causing the above) is dominant enough. Not sure how you quantify that influence from perspective of the law though, but practically their current influence is very damaging for the whole Web.
