Consider a newspaper with articles and ads. The customer is absolutely aware of the attention currency when there are articles side-by-side with ads. And the customer may decide NOT to pick up the free local "paper" in the supermarket bin that's 80% ads with only a few articles.

On the other hand, when there are ads that are formatted like articles, but with a little disclaimer ("This is a paid advertisement"). Those can be confusing. Better newspapers will do things like using a black border or requiring a different font to make it easier for customers to tell the difference. Finally, there are articles written by the newspaper's journalists about a topic chosen by an advertiser -- those are VERY subtle and confusing to the customer.

I presume you can on your own extend this metaphor from newspapers to the Web.