Using a dark net market is criminal
That isn't always true because not everything listed on dark net markets is illegal.
In most cases people using dark net markets are just buying psychedelics, which I personally have no problems with. I'm pretty sure that authorities are well aware that only a very small percentage of the conducted transactions on dark net markets involve ID fraud, credit card fraud, weapons, etc.
It's the mainstream media that comes up with money laundering, terrorism, etc. Most people who are technically clever enough to use a dark net market know very well that they shouldn't mess with any of that while using Bitcoin, and they very likely don't. That's what the us dollar is for.
It's way more likely that the physical cash you hold in your purse is used to conduct actually illegal transactions offline, but no one seems to care because it's fiat.