If I argue my case to those who hold opposite opinions, do you think I just let those who are on the fence tilt to the side away from me? By winning an argument against a lost soul, even if the one I was engaged with doesn't see the light, any "undecided" people who are witness to the exchange will be convinced far more thoroughly than if I ignored the adamantly anti-weed person and only spoke to them.

The undecided observer will have seen two sides of an issue, and watched one side triumph over the other. This leaves them with the power to decide for themselves which person was right, and chances are they'll choose the one who came off looking like the winner.

To turn your analogy back on you, when a salesman goes door to door, they're actively out there searching for people who are not presumptive that they don't want his product. They try to find people who are "on the fence" about the product and make the sale, and they certainly never show any of the arguments against their product. They're out there as propaganda-men to get whoever they can to believe what they're selling is 100% worth it, and what reputation do they have? Annoyances who most people will slam their doors on, because if you spend time listening to them, you'll probably get hooked by their claims and form an opinion without any balance.

The trick to legitimize a cause is to renege claims which are contrary to what is, in marijuana's case, the truth.

This thread was about talking to cops, anyway and most of them aren't on the fence when it comes to drug laws. I pride myself on my ability to get irrational people in a place where they have to run away to keep from admitting they've lost, and my ability to get rational (but misguided) people into a place where they must admit they have to rethink their position.