German here. In the article it says the law is there to prevent terrorism. But 9/11 isn't such a big deal here in Germany (except maybe for traveling). The problem is that those people can be detained for "bevorstehenden Begehung oder Fortsetzung einer Ordnungswidrigkeit von erheblicher Bedeutung für die Allgemeinheit" - "imminent commission or continuation of a misdemeanor of substantial public concern." So they are arrested to prevent misdemeanors which only come with a fine or a super short jail time.

IMO the real reason for this is to protect the automotive industry which is a large part of the German economy