There's a lot of skepticism surrounding the alleged connections between drug trafficking and terrorism. Perhaps rightly so: shortly after 9/11, the Bush administration backed a Super Bowl commercial pushing the connection between drugs and terrorists like Osama bin Laden, much to the consternation of pro-legalization advocates. "It's a cynical, cheap shot to take in the current political environment," said one New York-based advocate of drug law reform. "To make it sound like a kid who smokes pot is responsible for putting cash in the hands of Osama bin Laden is ludicrous."
But the connection, however tenuous, is real. No, your average pot-smoking kid probably doesn't contribute to Bin Laden's riches any more than your average SUV user; but there is real concrete evidence of drug trafficking and terrorism being linked.
The reason they're usually connected is, quite simply, a matter of desperation. Terrorism tends to stem from frustration at a system that is not allowing for advancement; when terrorism fails and ideology takes a back seat, and reality hits, drug trafficking enters the picture. It's an easy way to make quick money, and it's never difficult to find new recruits to peddle your drugs. Life is hard for most people, and a desperate and exploitative person can easily find a way in. Traffickers can easily utilise the same business connections and the same underground routes that terrorists use, much in the same way an old boys' network might operate.
Look at the FARC in Colombia, which has declined in recent years in part because of dwindling ideology. Around the late 90s, it became clear the FARC's Marxist ideology was no longer carrying it forward, and that it needed to turn to drug trafficking to support itself. Already a designated terrorist organization, the FARC was now also a drug trafficking organization (or DTO, in US authorities' parlance).
Interestingly, when terrorist groups turn to drug trafficking, it apparently makes it easier for the authorities to apprehend their leadership. Since roughly 2002, US authorities have extradited dozens of senior FARC members to the US on drug trafficking charges rather than terrorism charges. Likewise senior drug warlords in Afghanistan have been brought stateside for trial on drug charges, rather than terrorism, even when they were implicated in the former.
None of this really changes the fact that U.S. and Western European consumers are responsible for the drug intake, rather than it being a national security threat, but that said, the connection between terrorism and drug trafficking is very real indeed.