The real reason behind this fear is that some consider Satoshi their hero and icon. Anything, therefore, that he doesn't or wouldn't (if you maintain this isn't the real Satoshi) approve of especially as it relates to himself inspires strong feelings and irrational justifications like "he could be kidnapped and tortured."
Or at least that's the only reason I can imagine this argument keeps coming up. Any other wealthy person could also be kidnapped and tortured. Heck, most wealthy (100m+) people probably have pretty valuable things just laying around their house that are far less traceable than Bitcoins.
Of course, in this case, I hope the bitcoin network would resign itself to coloring those coins, if Satoshi was extorted out of his coins.
I agree there's (hopefully) a good chance his BTC are in cold storage in a safety deposit box, or something. People fail to do that with BTC more often than people fail to put millions of USD in the bank, though.
Actually, I've been wondering if he didn't withdraw from BTC because he was embarrassed about having lost the private keys to his early accounts... :-P Not that I really think that likely, either.