Yeah, doing it this way they can never make API incompatible changes without also fixing everything downstream... which effectively means once a library is popular enough it is locked at it's current API forever.
BUT this allows you to pay the prices of versioning (downstream burdened with rewriting => they never do => old code lives indefinitely) only in the worst cases. If done right (lots of tests, great CI infrastructure), fixing everything downstream is practical in many cases, and can be a win.
A subtler question is how this interplays with branching. You can't be expected to update downstream on every random/abandoned branch out there, only the head. Which deters people from branching because then it's their responsibility to keep up...