Well that's just it, my understanding is that FTL hasn't been proven to violate causality, or that causality is inviolable. It's just very strongly hinted at.
In special relativity at least it's pretty clearly the case that communication outside the light cone (so faster than light) will result in events happening in the wrong order in some frames, violating causality. I will not speak of general relativity, as while I've taken a course in it, years later I have returned to considering it largely dark magic.
On the other hand, if it were shown to be possible it would be rather disruptive to many other presumptions in physics.