You are absolutely correct. However, at that point it becomes an ethics issue. If these are financially motivated search results without disclosure then it becomes a whole different ball of wax. If they are deliberately screwing up their relevancy algorithm in the name of a bottom line then people need to know that.
The reason people trust and believe the spam search results that come up and blindly make purchases on those sites is the same reason they can't realize that Google isn't some non-profit public service but is in reality an ad-littered money machine: people are stupid.
