Over two years ago, Apple allowed transfer of everything from one account to another, once only per account, from iTunes. (I read it somewhere and made a note wishing one day I could use it if I had to.) They wiped out this flexibility.
Anyway, iOS and OS X are designed for single account only. Maybe that's the limitation of DRM, or Apple just want more sales.
What I think is: Apple doesn't want a potential second hand market, esp. apps, It's more a business strategy than a consume-driven / technical issue.