buy drugs using bitcoin was illegal, right ? 
It is like asking if killing people with knife is legal but killing them with gun is illegal. Buying drugs is illegal in general, it does not matter if you buy it with credit card, paper money or digital currencies.but in darkweb, some website only accept bitcoin,
No cash, No paypal, No mastercard because bitcoin is anomity
look at this coment
I bought some oxy and cokes from darknet. Bitcoin was the only mode of payment there.