It is quite easy to criminalize distribution and possession of digital assets.
Also no one care if you bounce bitcoins between your own wallets, any governments just needs to regulate at the border: retail shops that accept crypto and exchanges of crypto/fiat are very easy to identify and regulate.
It is already a crime to passively posses certain combinations of bits (i.e. that directly or indirectly [zip, truecrypt] represent a child porn image).
It has been made illegal to possess the implementation of a mathematical function (i.e. Kerberos being a "Auxiliary Military Equipment"). It will be pretty trivial for the government to make it ilegal to download, posses or install a program (e.g that interacts in some way with the Bitcoin network).
