One place where I think most people don't mind a little plagiarism is in joke telling. I can't tell you how many Dad Jokes I've found on the Internet and passed off as my own, and I think there's sort of an understanding that if you read a joke off a Laffy Taffy wrapper, you now get to tell that joke to others without attribution.

That said, in the professional comedy world, there's the idea of joke theft https://en.wikipedia.org/wiki/Joke_theft and I think most people recognize that as more clearly an example of cheating.