You must be laughably sheltered if blocking traffic as a form of protest is something you consider to be a “serious crime”. And yes, climate terrorists is kinda the next consequential step if you make protesting illegal.
It is in the fucking StGB. Neither you nor me define what a serious crime is, the law does. Do I agree? No. But it is still the law.
Every single protest blocks traffic my dude. And no, not everything in the StGB is a “serious crime”. Learn some nuance.