Your view just plain scares me, there wouldn't be so much heat over this right now if it was something the majority of people including app developers thought was the right thing to do. There's plenty of examples of things that I feel is uncool, but I can understand it from the business side. A few weeks ago, when people went nuts over the possibility of ads showing up in peoples instagram feeds, I was like, fine. In every single terms of service that I have ever read, that line, "we reserve the right to alter these terms of use at any time", appears in some form or another. That is not an end all, and it's not an important distinction. What you alter the terms to still matters, especially on this big of a scale.

You make it sound like if they did this and didn't tell anyone about it, just opted everyone in, and started selling off photos. That you'd be ok with this. Not sure where you draw the line, but facebook/instagram finally decided to cross it. Users of products do have rights, and honestly app developers should head this as an important warning/lesson. You should always have a right to protect the interest of your product, but sometimes, when you decide to monetize a service years after its conception, you mess up. Even facebook/instagram can make huge mistakes. Instagram is going to take a gigantic hit from this.