I couldn't understand what is the reason of this restriction. I tried remove it manually and I faced with situation that this restriction didn't need (not required). Finally all works fine without them.
But I see a big disadvantage in this restriction (new()). I can't create entity with protected default constructor for encapsulate logic for object creation into another constructor. It is so useful if you use style like DDD.
I suggest remove this restriction at all. I can provide pull request with fix.