Pinterest is the absolute worst. They're basically plargists at this point. I really don't like rehosters.
It’s less about plagiarism, more about a dead end vs. a stepping stone.
All things being equal, Pinterest should have been penalised into non-existence by now?
This doesn't always help you get to the bottom of things, but it is a next step to take.
I'd want to assume that their systems are more advanced than that (or if they're really in cahoots, maybe Google crawls them with a unique, secret bot UA to prevent this sort of thing), but I just got done with another thread where an ex-Tumblr dev said all user content was in a single S3 bucket without MFA delete so my gut feeling is lending a bit more weight to the "everything is held together with rubber bands and duct tape and it's only a matter of time before the jig is up" parameter than the "large, powerful companies tend to hire smart people that communicate and design robust systems effectively" one.
That's perhaps the kind of thing google would find quite useful.