Before you can understand why faster than light seeing is paradoxical, you first have to understand that the concept of simultaneity doesn't follow your common-sense notions under special relativity.
However, special relativity is based on two founding principles:
1. Experiments carried out in any non-accelerating frames will always yield the same results (this means that if you're inside a sealed up box with no way of observing anything outside, then you cannot perform any test that will tell you whether that box is stationary or moving in a straight line at a constant speed).
2. Any observer measuring the speed of light (providing she does it accurately) will always obtain the same result. The measured speed is affected by neither the motion of the observer nor the light source.
The second principle is far from obvious. In fact no one believed that this would be the case, and it was only accepted grudgingly after repeated careful measurements showed it to be true. Scientists were trying to measure the 'absolute motion' of the Earth by looking for discrepancies in the measured speed of light in different directions, and were perplexed when they could find no differences.
Einstein's genius was to accept that the measurements were correct and work out the implications. Everything followed from that and he had to abandon many notions that had been accepted for thousands of years and which were 'obviously true'. For example:
- there is no such thing as universal time, and it's sometimes impossible for observers moving at different speeds to each other to agree on which of two events occurred first.
- If you take any object and accelerate it up to a fixed speed, it will then be heavier and shorter than it was before you accelerated it.
- moving clocks run slower than fixed ones.
- matter can be converted into energy and vice-versa: E=mc2.
Now special relativity has been shown to be true by countless experiments and technologies. If you accept that it's true, then I can show you how faster-than-light seeing would violate causality (to some observers, events would happen before the thing that caused them).
But the thought experiments that illustrate this typically involve moving spaceships (or railway carriages) with observers at each end and a light source at the centre. Peacegirl would never accept the part of the story about which observer sees the light source first, so it would probably be a waste of effort typing out the explanation.