Tieflings can be demonically lineaged too. You have to put some strain on your suspenders yourself when you fluff your character, you don't have to abide by the letter of every character generation option. They'll never be able to include entries for every possible way a player might want to write their character.
Still, I guess a "Demonic" subrace Tiefling entry would be nice, even if it's just a generic one.
