This sets a very poor precedent that will likely haunt 5E to the end of its days and somewhat hasten that end, I would say. In previous editions, it was always possible for a specific setting to require the usage of certain otherwise-optional rules.
This decision essentially suggests that no official setting can ever now require an optional rule, however much sense it would make, however fundamental to the lore of that setting it is. At the very least it says that of Feats, which is pretty bad.
I'm sure some people will defend it, just as some people defend every Wotc decision until they hit that one they can't stand, but it was a bad decision, and smacks of corporate meddling, where branding and mindless consistency trump setting lore and and common sense. It's not 4E FR bad but it's the same kind of thinking, just applied to mechanics. In this case someone higher up the food chain at WotC obviously overruled the actual writers and decreed that Feats couldn't be used, and to the nine hells with the setting and lore consequences, just as someone at WotC once decreed that Tieflings, Dragonborn et al need major, world-altering, generic race lore consistent presences in the FR, and to hell with what needed blowing up to achieve that (they could easily have been inserted without blowing up the FR, just not in a way easily consistent with their generic lore).
What next, Dark Sun without Defilers, because that's not how it works in the PHB? Or without Psionics, even? (as an aside this PHB+1 shenanigans means any Psionic classes must appear in the Dark Sun book itself, which I suspect may well lead to WotC skipping both DS and Psionics in 5E, which would be shameful).