Let me start by saying I don't dislike Greyhawk. I play in a current AD&D 2nd ed. game set in that setting, Greyhawk and Blackmoor also, as the settings designed by the game's two creators, Gary Gygax and Dave Arneson respectively, deserve a place of honor among D&D settings.
Since Greyhawk, like Blackmoor and Mystara, have largely been ignored by the publisher for decades now and exist in a state of publishing limbo, I see comments now and then from Greyhawk fans lamenting the theft of Greyhawk adventures, NPCs, deities, etc by other settings. I'm looking at you Forgotten Realms. Most recently I was enjoying the first couple episodes of the #Greyhawk Legends & Lore podcast (http://greyhawkery.blogspot.com/…/legends-lore-premiers.html has the details) where the hosts briefly bemoaned the inclusion of some iconic greyhawk adventures (Hidden Shrine of Tamoachan, Temple of Elemental Evil, etc) in recent D&D 5th edition adventure anthologies. That gives me a little pause.
If you're a Greyhawk fan upset by the pilfering of your settings content by another, I can only hope that you have a similar regard for Greyhawk's theft of the Classic D&D adventures B1, B2 and X1 by Greyhawk at the expense of the Mystara setting.
Yes, it's true that B1 and B2 were published before any setting officially existed as a product from TSR, and early printings of both adventures contain some minor references that apply to Greyhawk, but in that same regard, much of the nomenclature in AD&D and (WotC 3e - 5e) D&D game manuals is shared with Greyhawk. To the average fan, which takes precedent? Vecna, Tenser and Mordenkainen are as much Greyhawk now as Quasqueton and The Keep are Mystara.
Then there's the Isle of Dread. X1 is not just a Mystara product, it is arguably THE Mystara product. That adventure, and the Expert rulebooks it accompanied in the 80s, established and created the Known World setting for D&D that later became Mystara. The setting is implicit and integral to the adventure.
So, in short, please, let's be fair. If you're complaining that the Forgotten Realms is stealing your Greyhawk material, please, also reject the Return to the Keep on the Borderlands AD&D adventure that appropriated B1 and B2 for Greyhawk as well as the Savage Tide adventure path from Dungeon and Dragon magazines that relocated our beloved Isle of Dread to Greyhawk. Show Mystara the same love and respect Greyhawk gets.