Why?
It's their station. They can do anything with it they want.
How would you feel if a competitor decided to do the same thing with Fahrenheit 9/11?
Moreover this is an attack that is not related to issues but is personal against a presidential candidate's character and activities. Maybe it's just that it's an unorthodox format. If it were part of a newsmagazine or news broadcast, it would be a little less conspicuous.
Again, if this were purchased as an infomercial, I have no objections at all. Even this way, I can only say I'm disappointed.