It doesn't belong to you. Taking something which doesn't belong to you is theft. It really is that simple.
Yeah, tell that to Mike Hearn and his merry band of blacklist supporters.
He's still pushing that? I thought the community gave a resounding no to anything that threatens fungibility.
My favorite post regarding that argument is this:
Mike,
Re: defence vs attack (hmm am I still allowed to use that spelling?)
It's more subtle than that I think. What we have here is the issue of layering. In the world of banks, it is also the case that there is such a thing as irreversible payment. But it is hidden behind many layers and only accessible to the most powerful financial institutions. Even a SWIFT international transfer is pretty close to irreversible; but it is also heavily censored. That network can only be used if a big list of conditions are met. And above that layer are the real "hard money" layers (to the extent that any fiat money can be "hard"). Interbank settlement (often on a batch basis e.g. Euroclear and Clearstream) and RTGS (where it exists).
So the everyday experience is one of interacting with non-real time and reversible payments. Clearly the average punter wants to use that kind of system, including its time and cost payoffs, even if he's not aware that he does. Because he wants the right to get his money back, which he doesn't always get, but he does in the most clear cut cases of fraud.
There is no reason a Bitcoin economy cannot have these layers.
If people cannot be bothered with the complexities of hardware wallets etc. , then payment services can be provided to them fully featured. Bank accounts, fraud detection, insurance, transparency or privacy in whatever blend required. All of that is possible, and what's more with scripting Bitcoin can offer either more lightweight or cheaper or just out and out better versions of at least some of these features. The attack you refer to - whether it be by law enforcement or by all kinds of other agents, can occur at this level, and that's fine because everyone who joins that level buys into it.
But Bitcoin offers something else which is priceless to everybody - an intrinsically unpoliticisable base level of value transfer which is far more secure and far cheaper and faster than anything that came before - for those who choose to learn how to use it.
That can be used by individuals, corporations or governments whenever they choose to do so, because nobody can stop them.
If you try to mix in those higher layers to the underlying protocol, you destroy it.