In both cases there mere existence of the technology helps both good and bad parties to achieve their aims, that is all that matters.

Now, suppose that there’s a transaction chain which involves you – say said local dealer buying from one of the few other people using it — and how you’d prove that you weren’t a knowing participant in some sort of laundering scheme or deal. This is why tumblers aren’t likely to be used by most people and why running a Tor exit node is so risky: you have to worry about being charged with the worst thing done over the network and proving that you aren’t just trying to look like a naive user.

I think that the ZK (ZCash, AZTEC, etc) technology will solve this problem and we will be back to the case where the government wants to make the technology itself go away.