If you examine it carefully, you'll see why. (Click on image to enlarge)
Waterson refused to commercialize his creation in the way cartoonists were expected to. Unlike, say, "Peanuts," there are no plush toys, no TV specials, no Hallmark ornaments, and no car stickers. That last is important because a bootleg stickers featuring Calvin urinating on an automobile logo became -- and remain -- quite popular. You can get them with either Ford or Chevy as the target logo -- for your Chevy or Ford pickup respectively.
And that's the genius of Pastis' sequence. He's the only cartoonist I know of who references that well-known but never talked-about use of Calvin's image. Note the sign's placement. It's on the far right of the panel, and halfway up -- ensuring it's the last thing the reader's eye will see as it tracks across the panel. The large joke is the grown-up Calvin hawking his childhood celebrity. The real punchline is the suggestion that he's behind those ubiquitous truck decals.