Maybe they are only going after exclusive content (which they do have) or they employ some sort of digital watermark - which wouldn't be hard to do at least for un-compressed images but they would have had to start doing this from the beginning which I doubt they did do or they would have been going after infringers earlier. In any case going after non-exclusive images that have been out in the wild for ages would be pretty hard. Although they do have a record of all the refunded image sales, so maybe they can go after them if they are still in use. If I had an illegal use I'd just say I bought it from Lucky Oliver or something like that.

In my very limited searching for my own images it seems like sometimes there was a legitimate use - often with a weird crop, and then a heap of copies of that very same crop - often in blogs that were not in English. I don't see them collecting much from those places even at under a dollar.

BTW, love the name of this publication

I bet they copied that from somewhere...

issue will be if infringer don't know which agency they actually stole image from.  If they stole it from a licensed use, they probably don't know the origin.  If they pay they will now have a licence, but what makes it valid?

Legit client downloads image available across all MS from SS
Organisation "steals" image and makes a use it
DT comes in, says do you have a license?  No, bad bad person, here we can issue you one.
 
OK fine, i'll be honest, here is money.

Is this valid license?  What if SS gets in game later, and has a way of knowing it was Their original download that was stolen....

I think as long as they charge something more than regular use and especially more than subs prices this is probably a good idea (and pay the artist enough), otherwise any user could just figure IF they get caught they can just buy a license, so why bother up front.