the problem is that it's an unprovable assertion. nobody can tell whether you've kept the private keys. all a thief would have to do in your place is keep all the data, inspect the block chain periodically for a very large balance, and strike once many years from now in an entirely untraceable way.
Of course. YOu are 100% right. But what is different here is my ass is on the line. Thief should plan on his ass being kicked if he is giving out his real name and address like I am. MtGox and MyBitcoin could stiff you any second, and your bitcoins could disappear if they get hacked (as has already happened to some).
You run this for several weeks, months, years, and keep tabs on how much money is in the paper. You get bored and decide to take it all and move to another country. We have your thumbprint, woopty-fucking-doo.