I would argue that advertising can make me
want something, but it's still up to me to
choose whether to buy it or not.
When I look at a typical beer commercial, for example, I can see that they're trying to associate girls in bikinis with drinking their beer. By making that conscious observation, I can interrupt the association, and then
choose to accept or reject it. I find that analyzing what a commercial is trying to do to me takes a lot of its power away.
I agree with you that advertising is an excellent way to illustrate human behavior. We all have things that we do automatically, without thinking or questioning them. Advertising attempts to reach us in that state, and I think that passive, thoughtless entertainment, such TV, is a very effective way to reach that state. A person could live his whole life with nothing but conditioned responses. However, we also have the ability to question decisions, think through the consequences, and then deliberately choose something.
As another example, addiction is one of the most powerful ways that someone can lose their free will. Most addicts have lost their ability to choose not engage in their addiction. However,
mindfulness is becoming increasingly popular in addiction treatments. At the core of mindfulness is the exercise of thinking ahead, and deliberately choosing.
I think addiction and advertising both illustrate the same point:
Free will is available to us, but it's optional and not easy to use.