There's a piece missing from your logic. Exactly what does Apple have to fear? That they won't become the Dell of tablets? I don't think they're afraid of becoming the maker of the fashion statement tablet -- that's where the high margins are! A killer app (with healthy competition) that benefits all tablets helps them. A killer app that's a monopoly can hurt them, because the killer app can disappear from their platform.
They don't want a third-party app to become a key part of the iOS experience because then when it is ported some of the Apple sheen would likely rub off on Windows/Android.
If there's a killer app on their platform, then Apple can't change their platform in a way that breaks the app. Consider the hoops Microsoft has had to go through with Windows and IE to avoid breaking important third-party products. Since the whole point of Apple products is high-end, uncompromising design, letting killer apps get the upper hand would quickly make iOS stagnate and lose relevance. Once that's happened, the only way to get ahead again is something like the MacOS-to-OS X upheaval, which wasn't particularly pretty.
