Mobygames and TheLegacy added watermarks to their box scans ages ago, probably for the same reason. I guess we have to live with it.
For me it's a bit sad. I made over 500 PNG game icons, mostly taken from HOL box scans, which aren't available in the brilliant exoticon collection.
I wanted to do some more, maybe converting them to OS3.5/Newicons too. It's hard to remove a watermark without destroying graphics.
I understand ZetrO, looks like he used important parts from HOL box scans for this "Custom CD32 Cover" project. But with watermarks it's much harder for him to get good source material for his work.