Causality, though, is a postulate in the system. If you start with the assumption that faster-than-light is impossible and build a system around that, the system you build around that assumption will always prove that it's impossible -- until something else proves otherwise.
Causality it NOT a postulate of the system. General Relativity fully allows for universes in which causality does not hold. Such universes would either turn out to be either very strange, or actual use of FTL drives would have to be extremely rare.
