I think the founders of e-gold and LibertyReserve can tell you all about how hard it is for the US to regulate financial services conducted over the Internet using databases and tokens but over the planet by actual people and corporations.
Apples and oranges. You can't sieze the server or domain name and turn off cryptocurrency. That guarantees that the back end keeps running. Then your only way is to regulate the usage of it, but when that can be obfuscated with things like tor, VPN, public WiFi, etc it's going to be incredibly difficult to do any sort of meaningful enforcement. Add decentralized 'smart contracts' such as what ethereum has and it can be run beginning to end in a fully decentralized way.