Well, who said it has to be fair?
But seriously, I didn't make this rule, I'm just quoting it. But i understand the reason behind it and I agree to it.
I personally think the difference here is that Twilicorn actually happened in the show and is now part of it — no matter what haters think.
Humanizations and anthros on the other hand are pure fanmade ideas. (I know about EQG, but most humanizations are not EQG-style. Not to mention that EQG receives just as much hate as other humanizations…)
I personally don't have anything against humanizations or anthros, I enjoy them as much as I enjoy ponies, but i understand that there are people out there who prefer to keep their "pony-expirience" as close to the show as possible. And if they use filters to lock out humanizations, and then still see them as featured images, then they'd be pissed. And they'd have all rights to be honestly.