Interesting - good post.
I actually agree that they should be able to voice opinions freely - 9 times out of 10, terrorist groups arise from agenuine grievance. If the matter can’t be discussed (albit by political rather than voiolent participants) then you effectuively aid extremist recruiting because the only remaining resolution is violent.
I remember the father of one of the Omagh bombing victims condeming the ban on such speech by terrorists, saying he wanted to hear what possible justification there could be for killing his little girl.