Part of the agenda of 'terrorism' is create the over each of .gov and other organizations to change our society and norms.
Not so worried about instructions to make explosives. As you noted, that stuff is all over the place. More worried about suggesting exactly how to take down airplanes. Also, Anonymous isn’t government, so there are no 1st Amendment concerns.
Finally, our society and Constitution are not supposed to be suicide pacts. Striking the enemy - even if we have to break some otherwise reasonable rules - will be required or we will lose.