I would at least sort of understand the reaction if it was a free app that offered paid DLC, since a lot of free apps are free, through and through. But it's a paid app, with paid DLC. HOW IS THAT A PROBLEM TO ANYBODY?
People can have a sense of entitlement when they pay for something. They'll say "I already paid for the game, why do I have to pay for it again?" And it's not entirely unheard of, giving out all future updates for free has always been part of the Minecraft model. So they might be used to paying once and getting updates and new content free.