I suppose the big difference is that no one owns Romeo and Juliet. The performance of a 1950s hot rod version of a Shakespearean work does not in any way prevent another troupe from performing it in a way that is more faithful to the original work. The refusal of WotC to open the Greyhawk setting on the DMs Guild, however, means that even as they take the Elemental Evil story and twist it to fit the Forgotten Realms, and even as they smush Tomb of Horrors together with the Forbidden City and relocate that, too, to the Forgotten Realms, it is expressly forbidden for anyone to publish an update that adds some content to one of these classic modules or adventure paths and update the encounters and whatnot to the current edition. Mind you, you can absolutely publish your adaptation and enhancement to a classic Greyhawk adventure on the DMs Guild, as long as you set the thing in the Forgotten Realms (or Eberron, or some Magic the Gathering setting--yes, MtG is DMsG legal while the original published D&D setting is not!)
I really don't mind that they write all the new stuff for the Forgotten Realms by default, I'm happy for the kitchen sink to get all the scraps. I do, however, object to the fact that other settings (specifically Oerth and Athas) remain locked off from 3rd party work.