All iOS jailbreaks are a result of security vulnerabilities, which Apple tends to fix almost as soon as they're discovered - and ultimately, it's Apple's responsibility to make their sandbox secure, regardless of what's running in it. I also don't see how installing outside apps would make jailbreaks any easier, given that you can already connect your phone to a computer and temporarily install an app on it (and for people who are motivated to jailbreak, this isn't much of a hurdle).

I haven't done enough iOS development to know for sure, but I'm assuming Apple could prevent private API usage by apps through technical means, rather than just app review.