they buy ads, just like any other banner you see on sites all over the place, but when the surfer clicks and goes to their landing page, they offer some incentive to the surfer for installing their software (just for eg. "see nude pics of Jessica" or "50 free arcade games") which the surfer gets access to if they install the toolbar
so to respond to what you are asking, a site itself is not "infected", it is a situation more like ads are placed on sites to entice the surfer to go and "infect" themselves
it is hard to tell which sites might have those ads on unless you boycott all sites that use those third party advertising networks that allow the toolbar-infecting-type businesses to place ads with them (sadly there are some pretty big ad networks that allow it)