It's not fully anonymous. Say my friend gets some bitcoins completely anonymously by doing something illegal. And then sends them to me completely anonymously. And then I go and spend those same coins on something like web hosting which ends up having some link back to me, eg I accidentally log in to it without using Tor one time.
The police can then trace that hosting account back to me personally from my IP, and then ask me who sent those coins to me. Even if I refuse to tell them, they can still look at my various contacts/friends/family to see if any of them are likely to have committed the original crime, and investigate further.
You can even connect to the bitcoin network via Tor. The risk comes when you spend them. If there's any connection between you and the recipient of those coins, then you're potentially screwed.
> The police can then trace that hosting account back to me personally from my IP, and then ask me who sent those coins to me. Even if I refuse to tell them, they can still look at my various contacts/friends/family to see if any of them are likely to have committed the original crime, and investigate further.
Has that ever happened? If there's no way to verify that a transaction was made it would not hold up in court. In the US for stolen property it's possession that's what gets you in trouble. If the cops find you holding a laptop that was stolen the day before it doesn't matter if you paid some guy off Craigslist for it, you're in trouble (at least out your money, but if you can't prove you paid for it you're looking at a felony). What they won't do is investigate everyone in your address book (no probable cause).
The police can then trace that hosting account back to me personally from my IP, and then ask me who sent those coins to me. Even if I refuse to tell them, they can still look at my various contacts/friends/family to see if any of them are likely to have committed the original crime, and investigate further.
You can even connect to the bitcoin network via Tor. The risk comes when you spend them. If there's any connection between you and the recipient of those coins, then you're potentially screwed.