Actually, Apple does try to catch those apps that sign out. In the process, they will miss the more cunning ones. And they make false positives.
One of my free iOS Apps, a RPN stack-based scripting language, came with some sample scripts you could in-click install (aka, move from App bundle into the sandboxed Documents). The App also had a button linking directly to a Wiki explaining the language and had copies of the scripts.
Somehow Apple evaluated that my App downloaded the sample scripts from my wiki down to the user device. They considered it violated the TAC concerning the "download of executable code" and had refused my original App submission. No amount of debating on the phone with Christ Whats-His-Name resulted in him even wanting to listen, giving me the cold shoulder. I shrugged it off, reworked some of the app and shipped.
Many revisions later, the App actually still has some sample scripts that it "auto installs". Just no longer has the Wiki link directly (it has a script to access it and now, inline manual).