The big psychological difference is that they are taking something that was free and then starting to charge/advertise versus having it be that way from the start. Kinda like how people would get annoyed if they have to pay to use the bathroom, or for napkins at a restaurant. Or checking you bags on an airplane. It's about setting expectations.

The other thing is that a lot of people use Instagram to share photos they only want their close friends seeing, so people are more sensitive about privacy changes.