Tiefling as a core race?

[Zander]

Besides, its not like its hard to say "Nope, not in my game" to anything at all, Core or not.

Only if you're the DM. As a player, your ability to shape the game world is rather limited. So if you want to play in a world without tieflings, you have to find a DM who agrees with you. That might be quite difficult in 4E because, as seems likely, most DMs will handwave the PHB races - including tieflings - into their game.

If you hunt around, you may find a tiefling-free game. But you'll be slightly distanced from mainstream D&D gamers. If you go to conventions or join an organization such as the RPGA (or its successor), it's unlikely you'll find a game that suits your tastes.

For the record, I'm ambivalent about tieflings as a PHB race.