What makes joke “theft” on Twitter egregious is that simply retweeting the source would be easier, while forestalling any accusations of plagiarism. It takes extra effort to obfuscate the author’s identity by cut-n-pasting, and it’s evidence of bad intent.
Whether a one-line quip is substantial enough to deserve copyright protection is another matter. You generally can’t copyright a three-word phrase, but on the other hand, Ogden Nash’s poem “Fleas” probably should qualify, right?
Adam
Had ’em.