Because someone in a position of authority at Amazon made the decision. I know people want to hear something enlightening or that even makes sense. That's not companies today work. It's always some absolute fucking brainless suite that storms in and demands these changes are made because they themselves, despite not knowing shit about anything, thinks its a good move (for very very stupid reasons).
And this isn't DRM. You gotta learn what that acronym means. This is just the way they scale their service. Has absolutely nothing with digital rights or copy protection.