If a user comments on a website, and a third-party business owner uses text from that comment for commercial purposes, is that plagiarism or theft?
Plagiarism is not a legal concept. However, commercial / non-commercial is completely orthogonal to plagiarism, as is permission. Instead, plagiarism is all about (a) correct punctuation (things in quotes) and (b) proper attribution. If Smith copies a slogan from a website and uses it, passing it off as his own, then that is plagiarism. If he says that this slogan comes from someone else, it isn't. Plagiarism is about social sanctions, not law.