Correct, it is a valid point. But that does not mean you should post it everywhere, especially considering this user is not intending to cause harm.

Stopping plagiarism is good, but do not alienate those who are genuinely good people and made a mistake. You are creating friction within the Steem community by polluting every single persons post who forgot to link a source. Say it once and then move on.

The situation is different if people are trying to make money knowingly from other people's work and you are doing a good job with that. But use common sense. Not everyone is bad.