The problem is that (as far as we know) we cannot determine when two (remote from each other) events take place relative to each other when we are constrained by the speed of light.
We cannot determine which occurred first. This is a limitation due to the information communication limit. So the theoretical basis (for consistency purposes) is that faster than light motion would violate causality.
If on the other hand, there is an absolute frame of reference, then all events can be related (in time) to this absolute frame of reference. In this case, no matter how much faster we can move above the speed of light, no violation of causality would be possible, we could always determine the relative positions in time of any and every event, no matter how remote from each other.
Since we do not know (as in, we cannot determine if such a frame of reference actually exists) of any such absolute reference frame, we are unable to determine in reality what would happen in the event of faster than light motion. The mathematical basis of the prevalent theories give causality violations in such events.
The universe is much stranger and more unknown than what is purported to be understood. Always keep in mind that no matter what theories are currently in vogue, they are a very pale and simplistic understanding of the reality that we live in. Every theory we currently use is flawed and fails in some way. That is not to say that they don't have utility. They do, but they are also limited in that utility.
The problem that is faced today is that people (quite intelligent people for that matter) forget that the mathematical descriptions they use in formulating theories and models of the universe around are always simplified models. There are always simplifying assumptions made. Too often, these assumptions are so hidden that those promulgating the theories get so caught up in their theories success that they forget that it is a limited and faulty description of reality. This then leads to some of the fantastical beliefs that they will then propagate.
This effects things all areas of science and the upshot is that when problems are highlighted with specific parts of the theories in question, there is a great amount of effort undertaken to defend the theory instead of applying critical thinking to the problem and looking for better alternative descriptions.
An example is the Standard Model of Subatomic Particles. One of the basic assumptions is that the Big Bang occurred. This underpins the entire model. Yet how often does this particular assumption actually get stated as being a fundamental assumption for the theory/model.
In conflict with this is the Theory of General Relativity. One of the "results" of the GR Theory is these strange entities called "black holes". Singularities that are quite problematic in any finite universe. Simply put, a universe in which "black holes" exist cannot be started by a "big bang". The singularity that is the "big bang" won't go bang since the gravity is way above anything escaping.
So we have impasse and hence, we have illogical assumptions underpinning some of our quite workable theories/models at the macro and micro levels. We must always remember that, though our theories are useful, they are a very flawed and incomplete model of reality.
The other thing to remember is that it should be fun to try and discover more about our reality. Too often, many who support one view or another (in terms of what theory or theories are applicable - witness the conflict between string theory and non-string theory defenders), come across as being afraid of their opponents.
Having a robust defence of your view is fine, but when it descends to ad hominem attacks, then all I see is fearful little people and not intelligent critical thinkers.