I don't know much German, so I can't read the original article.

It would be helpful to know if by "they might otherwise engage in climate protests," the people in question had planned to just say things but otherwise stay out of the way, or if rather the people in question had made public their plans to break laws (like blocking traffic, which many climate protesters have been doing lately). In the one case there is no crime, and governments shouldn't be detaining people just in case they commit a crime later. In the other case, even if someone isn't a terrorist, planning on breaking the law is itself a crime, and it's not "preventative detention."