And any case, if your coins are from a legitimate origin because you trade in MtGox or whatever, how would you prove it when the exchange is closed and the transaction list are lost?
There are so many grey areas in there.
not to mention other obscure way that utilize crypto to hide your way, and not there isn't only monero, what if i'm running an exchange, and i create some obscure coin based on signature ring, and i use it all the time to hide all my operations?
impossible to follow all the shit about the crypto world
What does the law exactly state?
it depends on your country, some have restriction, some have rules like the "600 rule" of IRS(if you stay below $600 in bitcoin in the course of one year, you don't need to pay tax, or declare anything), some has banned it, but they are still using it and none was arrested for that, wierd...
what i believe is that many countries will not regulate it in the end, and if your country has heavy restriction you can always travel on one of those country and you will be free to use bitcoin how you like the most