On what basis are broken apps/games for iPhone?

Break the same. Is the service responsible for checking the "purchased" programs (installd). Next, bought the program people can copy and give everyone with this patch. This for most programs. But some authors have advanced the kick from the protection, the same bijev (ICQ) or Skype (stsuko. besplatny, but jail detectit and swears).
So it was in 2.x-3.x. Then I do not know, for it changed the platform.

The head has to always break, even if you don't break something. There are only General principles and approaches, but each application is unique, and therefore the set of vulnerabilities for everybody.