Nobody is forcing their customers to give them money (save possibly the cartels themselves, I'm not sure). They could go with out or go to a country where it's legal, among other options.
They had a choice the first time, at least, even if you claim it took that away after. It seems odd to blame one group of people for 3rd degree consequences when ignoring the rest.
That seems to be an unpopular statement, but it's still true.
It's like asking people to stop voting for the least-awful of the two candidates showcased on the teevee. Yes, they should just stop for their own good and for the public good, but it's not going to happen.
The blame was in a conditional statement: if you blame the law forbidding drugs, why not the people actually funding the cartels? They may not intend for such to happen, but neither could it happen without them.