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.