The consequence being that if we don't class this as plagiarism, we then open the floodgates to dozens of accounts copy/pasting single lines of text, then citing this example as precedent as to why low effort posts are permissable. It doesn't matter if it's common knowledge, it has been copied verbatim without accompanying independent thought or comment.
I agree with this completely. It doesn't matter if it's a headline or text from an article: if someone else wrote something somewhere on the internet or on the printed page, you have to cite the source.
The only thing that was written in the post that was reported as plagiarism was written by someone else. That would lead the average person to reasonably assume that those words came from the brain of the member who wrote them--which isn't true in this case, hence it is plagiarism.
But hey, it's not up to me to decide. See how the appeal goes, OP. You might get lucky, but use this as a learning experience if you get unbanned.