(googler, but i work on and far prefer iOS. i care about the platform and i wish it had actual browser engine competition)
FWIW I like Firefox or Chromium a hell of a lot better than Safari, except that both drop my Macbook's battery life by an hour or more versus Safari, and both feel heavier in terms of their effects on overall system responsiveness (the feeling I get is that Safari is way better at keeping JavaScript from stomping all over everything). I do use Firefox on Linux, if I'm on something beefy enough to use a really heavy browser and still be able to multitask.
For that matter, it'd be worth keeping other browser engines out if their integrations broke Apple's only-vendor-who-seems-to-give-a-shit accessibility features. I dunno, maybe iOS Firefox and Chrome already do, but I'd imagine forcing Webkit keeps people from embedding browser engines that screw with that. Which, again, wouldn't bother me if anyone else were half as good. Please, for the love of god, anyone, compete with Apple. No one is, right now.
[EDIT] and by Safari I mean Safari's webkit, of course. You can embed webviews! You can run JS! You can ship your own browser! You cannot ship your own browser engine, for a bunch of very good reasons, and I'm having a hard time in this case of thinking of any nefarious reasons for the rule.
[EDIT EDIT] and then of course a browser engine is a giant attack surface. Better one version of one browser engine on your device than 20 versions of 3 browsers, most of which anyone but a turbonerd won't even realize are there because they're included with non-web-browser apps.