Some portals have specific guidelines for political advertising.
Those guidelines may be good-faith efforts to set ground rules. But in some cases, they have the effect of diminishing the quantity and quality of political expression online. I have a hard time getting upset about banning negative banner ads on Yahoo. I think that moreso then on TV, ads do reflect on the site carrying them, as there is less seperation between ad and content. This lack of seperation causes problems as advertisers abuse this fact to trick people into clicking on ads that look like operating system messages or even a normal part of the site they are viewing.