You see them everywhere. Links to hosting providers, Amazon, and other referral programs. Ads, like Google or Yahoo, are clearly indicated as ads but Amazon links often aren't. Referral programs, especially for hosting companies, almost never are.
My question to you is whether there should an obligation from the site owner to disclose these types of things? Does it matter whether the site owner receives any financial bonus from including a link on their site? Does it matter if they make a full post about a product or service and receive a financial bonus from any link on the page? Does it matter if the product or service in question asks for and is willing to pay for a review on the site?