Google Play allows app with a private licensing system?
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I'm developing a Android app, which for some reasons need a custom licensing service. Practicaly the user download the app from market, and app will require a license code provided by me.

I have seen that some important apps, can be unlocked through a unlock code sold by other methods (paypal). 

This is allowed from Google play terms? I read in this question http://stackoverflow.com/questions/7054107/android-custom-licensing-server-does-this-go-against-the-market-agreement
which it violates the agreement, but then some popular apps are breaking the agreement and are not removed?