Amazing - these were Circle K chain stores selling these stickers. How was this not enforced?!

It's not like stickers are particularly difficult to make, or Watterson had an army of auditors combing every gas station or car meet looking for sticker makers.

They have (as I understand it) challenged and stopped some folks from doing things, but something like the Calvin sticker was pretty ubiquitous. Even then, some later ones were particularly bad Calvins.

I had a vinyl sticker of Spaceman Spiff on the back of my motorcycle helmet. I bought it at a motorcycle race back in the 90s.