The difference between not giving back to open source and over grazing is not contributing to open source doesn't actually hurt other. It's just not helping others.
Like, we've all seen the "have a penny/need a penny" trays at cash registers, and we all understand that sometimes we take or leave things there. Amazon is basically able to send someone to every single cash register in the city and take all the pennies. Sure, maybe that's not explicitly disallowed, but it definitely breaks the model.