Trademarks might be expected to get a few seconds of attention at a given moment, much of it subconscious visual processing.  In the 5-10 minutes it takes to read an Onion article, almost all people will observe something not right and be motivated to decide whether it's a fake story or not.  In the 1 second it takes to observe a trademark on a piece of clothing, most people will not have the motivation to decide whether it's a fake or not.<p>And even then, the trademarks are still very similar.  Even a person looking at them side-by-side can reasonably suggest that the South Butt logo is a humorous sub-brand that comes from The North Face.  Often, with Onion articles, you can point to some fact that contradicts fundamental scientific laws and therefore cannot possibly be a true story.  Spinal Tap is technically a real band, and while the story told in the movie is fake it is still plausible.  I would not be surprised at all if someone saw out-of-context clips of <i>This Is Spinal Tap</i> without knowing what it was, and assumed it was a real documentary.