You're probably right. I can tell with pretty great certainty say that there are probably 100 lbs of magazines on each plane, but if they weren't on the plane, they'd lose their advertising value. nobody would pay to advertise in a magazine that is kept at the gate. the value comes from knowing that someone sits in a seat for 2-6 hours and eventually their boredom will make them succumb and finally read the ads...
it would take a real bean counter though to do the calculations, and who knows, maybe that extra weight does mean the magazine isn't profitable if you put fuel consumption into the mix?