Hiring a hitman should be illegal.
Buying slaves should be illegal.
Bribing the court system should be illegal.
This is not to say that every restrictions is justified, but taking the stance that no restrictions are justified is advocating anarchy.
The corrupt judge and barrister are very much willing. The negative effect is borne by a third party. Same with crypto. The buyer and seller are happy as can be. Then the rest of us have to pay to investigate, extradite, charge and liquidate Bankman-Fried and FTX.
Cryptocurrency has unwilling victims as well, but the linkage between cryptocurrency buyers and ransomware sellers is much less direct
If you want a closer analogy, gambling is also something that happens between willing participants and is almost universally either regulated or outlawed.
There are some good arguments that we need clearer, better regulations for cryptocurrency exchanges and a lower barrier of entry for low income/asset investors into securities.
However, some level of regulation of cryptocurrency is clearly required and is not out of step how we handle other parts of the economy.
In which case, sure. Hence why the government regulates transactions.
Selling a slave should be illegal.
Accepting a bribery should be illegal.
Chop the trunk, not the branches.
You don't see any reason why hiring a hitman should also be illegal?
Thus you agree that there are things that consenting adults should not be allowed to do with their money.
Edit: There are good points about how much freedom it is worth giving away to make it easy to catch this crime, but I have a hard time seeing an argument for it not being illegal.