In the worst case scenario internet providers will block the website.
Why we see those things just on the Darknet ?
no if you really think about it you will see that it is not a good idea to use a currency that is not anonymous to buy illegal stuff and in real world people use cash to do this instead and trade in a dark alley in a van
and as far as online goes they use a real anon coin such as Monero to do the trade in Darknet.
with that said, people do all kinds of crazy things. nobody can stop them. there is no why in my opinion. they think it is anonymous, they do it and get caught. and it is not "just" on darknet as i said