I'm not sure what your point is. My best guess is that you're implying that the only reason people don't do things that are illegal is the fact that they are illegal. If I'm wrong, please correct me, and disregard the next paragraph.

I see two problems with this point. Firstly, while I'm not entirely sure how the legal system in the US works, but I suspect it would be illegal for Coinbase to shut down and run away with everyone's Bitcoins. I doubt that USD is the only asset that is legally protected from theft/fraud. Secondly, I suspect that Coinbase has a huge incentive to not do this even if it were legal to do so. Namely, they should be able to make a lot more money by conducting their business legitimately and continually than if they ran off with one lump sum.