User blog comment:Maki Kuronami/Extra Lesson: Attack on Grimm/@comment-24267097-20141023135454

"Meanwhile, capturing a Grimm is not a great idea, as the possibility of it escaping can happen... or it just apparently dies. Huh."

As a grimm learns while it is in captivity it eventually becomes smart enough to escape it's confines forcing its captors to do one of the following

1. reimprison it better

2. kill it

3. release a wild and dangerous Grimm into an unprotected area

This all of course makes controlling a grimm by confinement difficult.