There is a base cynicism you can have about Twitter, Facebook, and Instagram: their revenue comes from advertising, so their users are product, inventory. But it's more true that, in this particular miniature controversy, neither side is particularly aligned with anyone who lives outside their balance sheets. Instagram wants you to see Instagram photos on their web site, not through a Twitter preview. And they want you to join the service before they'll let anyone browse more than one photo from any one user. Twitter, similarly, doesn't want a property owned by Facebook, or any other tool for that matter, to make profitable use of its social graph. And Twitter would rather you not leave the confines of your stream to check things like Instagram photos out.
You can still use Instagram and Twitter for free, and have a fairly amazing reach with them, in some instances. But every time you see the word "discover" in their apps and their marketing, you should take it a bit less seriously, each time.