His consequence IS his brain injury. He is in that jail every single second of the rest of his life!
I had a long post for you but then I realized you are ignorant on the subject of TBI and you opened your mouth without thinking first. Otherwise you are saying that anyone with a brain injury, is autistic, or any other special needs who can't always control their behavior should be locked away somewhere so they don't say or do something inappropriate.
Well, not exactly, but sort of. If someone is violent, for any reason, then no, they should not be anywhere where they cannot handle the situation safely. Would you be OK with a man coming up and punching you in the face, for any reason? Or twisting your child's head and neck?
So, for making threats to blow a place up, brain injury or not, then yes, there should be consequences. Allow that person to travel with family, but have room checks, be on a watch list of some sort, etc. I mean, can you personally guarantee that someone with a brain injury won't follow through on something said? If that thought is in a person's mind to start with, who is to say that it won't develop into an urge to follow through?
As for being ignorant - pot, kettle. I've worked with your above listed population. I've been punched, bitten, and kicked. And have you ever been stabbed by a metal compass point? It hurts. So, if you know of someone with violent tendencies, and you insist that they should be allowed to continue hurting others, doesn't that make you responsible? I now some people can't control it, don't understand it, don't have the comprehension that what they are doing is wrong. But that still doesn't make it okay.