Blame the press. That’s the latest coping mechanism for General Motors, whose slumping share price and falling profits have generated a wave of negative media coverage.The final straw for GM was a withering April 6 column by the Los Angeles Times’ Pulitzer Prize-winning auto critic Dan Neil. On April 7, in a short-sighted move that’s certain to add to the automaker’s problems, GM pulled its ads from the Times. GM isn’t the first Fortune 500 company to retaliate against a newspaper’s editorial coverage by taking a punch at its ad division. But most companies understand the tactic just doesn’t work; it only generates more bad coverage. That’s why the media companies that Daily Variety covers — a thin-skinned group, to be sure — have rarely resorted to canceling ads as a means of protesting stories. Threaten, they occasionally do. But few have taken things any further than that. Most media execs understand a basic principle that seems to be lost on GM: Newspapers must preserve a firewall between editorial and advertising. Compromise a reporter’s editorial independence and the integrity of the whole enterprise collapses. There are other ways to voice displeasure with a newspaper’s coverage: Demand corrections, write an opinion piece or buy an ad refuting the offending articles point by point. GM’s decision to yank its ads comes at a vulnerable moment for newspapers and magazines. Shaky advertising sales at the L.A. Times — reflecting the dicey ad climate of print media everywhere — have been a drag on the financial performance of its corporate parent, Tribune Co. As advertisers seek other avenues to reach their splintering audience, they’re spending more money on things like product placement and Internet ads, and less on traditional print ads. But the rift with the L.A. Times only magnifies the problems at GM — layoffs, closing plants, dwindling profits. Those are problems the press corps can’t control, and shouldn’t be penalized for.