You can buy many things with Bitcoin like weapons and bullets and that is indeed quite bad but the world is also getting crazier.
Buying guns, bestiality porn, drugs and other banned substances using physical cash is the most anonymous option. No matter how much you mix your coins, you are likely to leave a trail in the blockchain, leading back to your main wallet. However, if you use hard cash, then no matter how much the cops try, they can't trace you.
some people seems to don't understand that and try to show people how bad bitcoin could be because of that, i mean that this can be used to all those illegal things.
Blockchain cant really detect any transaction that is being used illegally thats why people who do this loves bitcoin and they remain anonymous with it.
not anonymous but pseduo-anonymous