"steemit should reward original content and not become a repository for cut and paste news "

Fair enough, that's your opinion. My opinion is that there should be a type of post for external links much like reddit provides. Sharing news stories from around the internet is a large part of social media.

"So, what does @steemcleaners look for and tag as “plagiarism”?"

He's only providing snippets with a link to the full article not the full article in it's totality which is why he isn't being flagged as a plagiarist. The distinction they usually use when punishing users is if they were trying to deceive people and pass the work off as their own.