the ToS is between a facebook user and facebook, not the author of the extension and facebook. In fact, the author has no obligation whatsoever with facebook. If facebook does not like their site messsed with, they could detect and block this extension like how some sites block users with adblock running.
Yes. They can ban anyone they want. My question is how they know that Facebook user X and browser extension Y have any connection. If the extension's maker was marketing on Facebook, he should have expected this.
Meanwhile, yes, the TOS is between the user and the site. If Facebook wants to fight its users and say "if you don't send HTTP requests for our ad images, we'll stop answering your HTTP requests for our HTML pages," that's up to them.
I do think we need to push back whenever companies assert a right to control how a user views their site. That is fundamentally not how the web works. Every resource my browser requests, it requests by my implicit command to do so. Nobody has the right to tell me what to request; they only have a right to decide how they'll respond to my requests.