This is no proof. You're assuming that the products and services marketed have little to no intrinsic or social value, and that the consumers do not engage with the marketing critically--on any level. Certainly we all know some people who seem to act as automatically as this, but I doubt I've ever met anyone who truly does.

You could say the exposure is necessary for the idea to be held. With marketing this is, of course, true. And in other aspects of life, certainly none of us comes into the world every morning with a blank slate of a mind. But would removing every expression of an idea, remove that idea? Perhaps in a generation's time? (It can't be done, but it's an interesting question to ask, all the same.) We do know that there was a time when certain hates--homophobia, for instance, to use an example from upthread--did not exist, at least not in a human mind. Yet they're here today. Did they require public fora?

I can be exposed to an idea or an advertisement and subsequently walk away from it, having rejected its worth or its relevance to me. Without having done so, I cannot claim any agency in my thinking. Whether this kind of agency is worth protecting is another question altogether. The liberal ideas of free speech assume that it is.