Serious question: is it illegal to exploit a cryptocurrency flaw for profit (strictly at the protocol level -- not hacking someone's machine or wallet, etc.)?
The FBI has seized Bitcoin a few times. It's likely that stolen cryptocurrency will be recovered in the future because of chain analysis and exchanges like Coinbase that want to score points with regulators.
that was a huge exceptional case. most thefts are smaller and don't attract much attention. anyone who is smart enough to steal a lot of coin is not gonna dump them on coinbase. They are more likely slowly tumble them and sell them offline or on small exchanges.