>They can claim a terms of service issue and ban you from their service but they cannot stop you from developing software.
They can stop you from developing software for their platform or using their API or brand, which is what Facebook has done. They haven't demanded the developer of Unfollow Everything cease developing software altogether, only that they cease developing software for Facebook.
Also, Facebook is nowhere near unique in having similar terms for developers.
The issue was 'automating user interactions'. The demand is 'I agree to never again create tools that interact with Facebook or its other services'.
That demand is wrong. What if the developer makes an extension for some other web site and then FB buys that site? They'll go the legal route again. I don't think that is right.
They can stop you from developing software for their platform or using their API or brand, which is what Facebook has done. They haven't demanded the developer of Unfollow Everything cease developing software altogether, only that they cease developing software for Facebook.
Also, Facebook is nowhere near unique in having similar terms for developers.