Links are one of the largest untracked forms of online currency, and as soon as the IRS discovers this they will try to find a way to tax them. Every link is an indication of an exchange of real value, even if the parties are linking naively.
Here is one example where an innocent link exchange with no money changing hands is increasing the value of a site:
Site A finds a cool image they want to use on site B. Site A copies the image and gives credit to site B via a link. In this example, a link has replaced payment for usage of the image. This is a basic barter deal and no search engine can complain about the link exchange.