Obama Starting to Wake Up and Fight Back - CNN Calls it "Negative"

Why is it that everytime Obama fights back, it is framed as being "negative", yet the Republicans get away with 2 weeks of the most vile, hateful, dishonest campaigning in years (the worst since at least Willie Horton), kicked off with the vitriol of their Convention in St. Paul, and they barely get called on it? Anyone else see how the coverage is lopsided in terms of how the two candidates are framed by the media? This is why I always try to point out that Obama is at a disadvantage just by default... because his message...his actions are seen through a media filter largely controlled by Republican/Conservative gatekeepers in the corporate TV news business.

Could it be that these folks aren't crazy about the idea of seeing Obama win the White House, because they may lose their preferred treatment and their tax breaks, etc? So therefore, they seem to go much lighter on Republicans...? Could that be it? I noticed this in the 2000, and 2004 Presidential races as well.