The German military is not allowed to act inside the country. Only exception is natural disaster relief. We have special police forces for high stakes situations like terrorist attacks.
But when there is a fight between 20 well trained soldiers who just don't give a fuck, what is the police supposed to do? Besides, the army police has more leverage and can influence the soldiers career and stuff as well.
It's not an ideal solution. Having a foreign entity project power - yikes! (even if it is just for their own subjects) but it's a solution.