That being said, this in no way implies that people are fated only to have the ideas to which they've been previously exposed or can't actively search out and take on new ideas.
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.
This, again, in no way precludes people from activating their rational faculties and rejecting those messages. But in those cases simple repeated exposure is often enough to win them over.
Your assumption seems to be that consumers, or at least the vast majority of them, critically engage with marketing material. Psychological literature definitively doesn't bare out such an optimistic conclusion. So such an argument seems to me at the very least scantly informed, but more than likely simply an apologia for a cognitive bias that is pro-marketing and pro-classical liberal notions of human agency.