Well, they can, yes, but then again, if one takes the precautions - like for example using cold wallets instead of transferring from one exchange to the other, the chances of the government succeeding are zero. They might catch some Dude who wants to buy a shitload of weapons-grade plutonium, but I doubt that they have the resources to trace the little guys that like to buy some pot now and then. And the biggest problem for governments and banks isn't even some dude buying plutonium. It's that if crypto currencies keep growing at that pace, they will make their fiat money worthless.