Earlier quoted context omitted.
Buying drugs does fund those actions. Drugs being illegal are a major reason why, and why I support legalization. But that doesn't excuse anyone currently making a purchase today. Think of it this way, illegal images could be produced by advanced CGI, but aren't because that tends to be just as illegal. That doesn't excuse anyone who consume images not produced by advanced CGI.
People have the right to control what goes in their bodies. I don't have the ability to see if my weed is "violence free", because drug prohibition took that away. You can't hold me responsible for violence among marijuana cartels if you don't give me a clean source for my marijuana. It's like saying encryption is used by terrorists, so if I buy encryption I'm indirectly supporting terrorism.
There's no such flow from people who buy encryption software to terrorist groups.