Yeah, I'm wasting a lot of time myself dealing with these bastards. They are about as ethical as street muggers, maybe less since they hide behind the law.
Anyway, one really obvious thing about Masterfile is that they jack up the price of the one image that they threaten people about. The majority of their images are royalty free (RF) and cost $65 as a one time license. However, every once in a while you'll see one that's "rights managed" and cost something outrageous like $1,500. There isn't typically any appreciable difference in the images - the only difference is that the outrageously priced one happens to be the one they are threatening over.
In other words, they poke around (or maybe even deliberately release) images that people then accidentally use and suddenly jack the price up and start threatening to sue people.
You can prove this by looking around until you fine one of their "rights managed" photos. Do a tineye search on it and I guarantee you'll find dozens of unsuspecting users whereas their other images rarely come up.
It's really, really unethical. However, it's probably legal. That said, if they actually sue, I'd find it hard to believe a judge would seriously see the value of their images as anything more than $65 given they're almost identical to dozens at that price range.