I would have done the same thing. People are insane, can you imagine what they would do to the inventor of digital money? (I realize Satoshi is not that, but a lot of people will think he/she is.)
He's presumably a computer geek sitting on a fortune that can be stolen and transferred without FinCEN knowing or able to do crap. I think that's enough reason to keep his identity secret.
It's a U.S. organization that goes after financial crime world-wide. I just gave it as an example. Point being, many types of money moving require careful laundering to prevent traces for financial or other prosecution. It's why most crooks focus on trying to steal straight cash if they're here in U.S.. A guy with an easily-moved, unregulated, semi-anonymous stash of millions is a much easier target and the government might not be able to help him. So, more risk than usual and varies depending on his location.
Let's say someone stole Satoshi's stash. To convert it to non-Bitcoin value, they would have to spend it or transfer it. Wouldn't people notice that the famous Satoshi block is suddenly in play? And the point at which it is converted to real value is the point at which a criminal investigation would begin.
I don't understand why Satoshi's stash of bitcoins is any easier to spend untraceably than any other store of value.
Not easier to spend than stock, bonds, derivatives, and so on that most wealth is stored in? Last I heard, there was more liquidity and less auditing for Bitcoin spending in general. An expert on Bitcoin might know that particular block moving would be a high risk operation. Thugs reading about his identity, worth, and use of Bitcoin in criminal dealings might think differently. They might decide to go get his money.
That's the risk. Not sure of its likelihood in terms of a percentage or even verbal description. It's near zero while he's anonymous, though.