True, many things cause cancer, but some are worse than others. I wouldn't put smoking in the same category as, say, cell phone usage. But there does appear to be some link between cell phones and gliomas. I think the problem is that all the contributing factors have not been identified. That's why the nature of cause and effect between the two can't be discerned.
But vicnic's idea is interesting. Suppose the study didn't factor in the added radiation of wifi and other such devices? I don't often put my cell phone to my ear, but at any given time, there are at least 12 wifi networks crisscrossing my office. There are even more at my apartment. So I might not develop a glioma, but exposed to the same conditions, someone who puts their phone to their head a lot more just might.