This is one of the reason governments and police forces aren't trying to complete wipe terrorists groups and extremists off the internet. Because if their post

This is one of the reason governments and police forces aren't trying to complete wipe terrorists groups and extremists off the internet. Because if their posts are publically viewable, then it's easier to watch what they're up to and stop anything dangerous occurring as a result. If they take them down, then these people simply go 'underground' and becomes it a lot more difficult to check whether they're planning anything. Of course, there's also a downside (that a certain few people might be inspired or encouraged by extremist content they see online), but it's better for security to let such people give themselves away before anything can happen.