They are drawing a fine line here. Who knows what the government would consider as terrorist views now or later. Don't get me wrong, I have no use for extremists as they are describing, but how far will it go?
Besides, is it not better to "monitor" these sites and possibly gather usefull intel rather than close them? Also it's just going to be like playing Whack-A-Mole. Whack one and two more pop up.