"The whole point is that it's intended to be preventative," said Binnie. "Does the state sit on its hands until something terrible happens, and then prosecute if anybody is alive ... to prosecute?"

The whole point is that they have not been found guilty of anything. If you use Binnlie's reasoning, anybody can be detained because something terrible might happen. Thats going too far. It puts all power into the hands of those who act out of fear (whether or not it is valid). It also relies on information from secret sources.

You don't have to wait until something terrible happens. You can charge them with conspiracy to commit...

that is if you have any evidence that would stand up in court.