[uberPhil]

"Why won't they"

Because Instagram can make whatever TOS they feel is necessary when they are interacting with your data.

You are using Instagram's servers and products display data. Therefore they can, essentially, do whatever they want with said data if you agrees to their TOS. Why do people still feel they have some recourse when you are volunteering your data to a private company in a public domain?

[pyre]

  | public domain

That means something specific. It's probably best to use another term. People are not releasing their photos into the 'public domain.'