The question, as msantos points out, is whether bitcoins are money or goods. If they are money, then handling stolen bitcoins are not illegal, and the rightful owner of them doesn't even have the support of the legal system to get them back: http://en.wikipedia.org/wiki/Nemo_dat_quod_non_habet
I would argue that bitcoins are money, and not goods. Goods have use value, bitcoins do not - they only have exchange value. They have no use in and of themselves.
TL;DR: Even if you can prove that someone else holds a dollar bill that was stolen from you at some point, you do not necessarily have the right to get it back.
I wouldn't say that is related to bitcoins (the currency). They've just put data into the blockchain. The tokens you and I trade as money (bitcoins) don't get a use value because someone has put ASCII art into the blockchain.
I'd argue differently because the bitcoins are destroyed in the process. In my view, they become the ASCII art. But I suppose it could just as easily be said that you purchased ASCII art hosting from the blockchain.
I see your point. I guess I just think it's insufficient that it has some use value, for it to be called a good instead of money. My point is that they are mainly (almost entirely) money.
I mean, US dollar bills can also be stacked together and lit on fire, thereby having use value as firewood, or you can write notes on them, making them a notepad, but they are still money because their exchange value completely dwarfs their tiny use value (if one can quantify such things).
UK law prohibits handling stolen goods. http://www.legislation.gov.uk/ukpga/1968/60/section/22