Yes? Are you afraid they can defeat you with tweets? The risk of normalizing suppressing speech is greater than the risk of terrorism.
The tweets are valuable propaganda to ISIS.
They are consumed by billions of young impressionable minds all over the world, inducing them to 'take up the cause'.
ISIS does not have the right to 'free speech'.
Stopping ISIS from spreading their selfie vids of how they burned people alive and 'You can too!' - is not controversial.
The vague, shifting notion of what constitutes "terrorism" and how that definition may be abused in the future, is.
Consider, if you will, what a modern-day Sedition Act of 1918 ( https://en.wikipedia.org/wiki/Sedition_Act_of_1918 ) would look like.
It does however provide a window into some of their recruitment behavior, as these types of outlets are the easiest places to lure vulnerable or impressionable people into deeper, darker corners of their propaganda networks. But I have to imagine that if you were to shut off their ability to lure people there in the first place it would do more to fight them than watching and following them.