If you want to sell me on the idea that tieflings are "just people", then make them a subrace of humans, otherwise I'm not interested in continuing this discussion.
Well, strictly speaking, tieflings from the fourth edition have become a subrace of humans. So, Bael Turath was a human empire, from where virtually all 4th edition tieflings originated. Nothing has changed in the fifth edition, where tieflings have human proportions, and non-human tieflings are not mentioned at all. Funnily enough, Pathfinder, who originally gave many pedigrees of tieflings and assumed that they could not only be humans, changed this in the second edition, making tieflings a subrace of humans. The same is true of aasimar.
Tieflings, and all Planetouched races, use to be assumed to be planetouched subtypes of human, rules-wise. As far as I know, at least. Fluff-wise there's always been the extrapolated existence of non-human tieflings, but whenever such a thing has been rule-ified it's usually under a different name -- Fey'ri (elf/demon), Whispling (halfling/fiend), Tanarukk (Orc/Fiend) for example. I am not sure whether or not these are direct equivalents to tieflings, though, but they're similar.
Anyway, this human-subtype assumption actually changed with 4th edition. Previously, tieflings did not breed true. Two humans could have a tiefling child, and two tieflings were more likely to give birth to an ordinary human than they were another tiefling. What caused Tieflings to be born was supernatural influences, or bloodlines that lay dormant and unexpressed for generations. But 4th Ed decided to make them their own, true-breeding race. Sure, they were still descended from humans. But now all tieflings had tiefling parents and made tiefling children (and they're all made out of ticky tacky and they all look just the same). I think it's pretty fair to consider them no longer a type of human but their own race entirely.
Very true, but because of the popularity of Tieflings (and seemingly for no other reason) they've been given very special treatment, to the point that I'm not sure they're can really be considered Planetouched anymore.
