I think the guy is right to be furious - the game industry depends on copyright and actively litigates copyright infringement. To do that while also being actively involved in copyright infringement is the height of hypocrisy, and fully deserving of sneering public ranting.
If you want to live by the "copying is theft" interpretation, you need to be prepared to be held to account for "stealing".
Yeah, if that map was free to use, then sure. But that's not the case.
It's become all too common I see this type of argument used in cases where big name studios rip work of others without attribution. This is an extremely weak (not to mention somewhat offensive) argument which is analogous to telling a rape victim "you should be happy the rapist found you attractive."
(marginally NSFW)
Sounds like exactly what he's doing:
"I am attempting to open communications with them now to ask where they came across the image, how it ended up in their game (with very pointed questions about their approvals and legal process), and what kind of compensation I can expect for the theft of my work."
Not exactly "childish" whining.
I think the childish part is yelling to the world his complaint. Sure, lodge a complaint with the company, then, if they ignore you, go postal on the internet, in the meantime, give the infringer the benefit of civilization.