I think it's implicit in the premise that only people who are interested in the topic would be prepared to pay anything to read something on it.

The charitable interpretation of his question would be "who, of the people who find this topic interesting, would not be willing to pay $3 for it?".

The question you are presumably structurally answering is "why wouldn't absolutely everybody in the whole world pay $3 for this?". In answering that, you were either being insulting to GGGP, or looking for a pretext to wedge in an "I don't care about this" comment. I suspect the latter.