Isn’t that all owned by people? How are they allowed to just take that (and likely from non-us residents as well)?

The relevant question is, who is going to stop them?

Just ask Kim DotCom about that. I mean, he sorta won extradition but the FBI did their best to ruin his life and we're pretty successful at that.

This is what it always boils down to. Right and wrong is irrelevant. Those with the biggest guns win.

... and those people are free to sue the Silk Road guy for the money they lost.

If you can prove your money is in there and can also prove that the money wasn't proceeds of a crime (selling illegal drugs, firearms, etc) then it's yours. I'm guessing very few people really wanted to tell the FBI who they were and what they were selling/buying on there.

how do you seize a brainwallet?

By cracking the weak crypto used to generate it