So Toughness gets triggered
Arguably, yes.
Of course, if you don't like that, then here's an alternate reading:
Even though Unresisted Drain = Damage, it's a conversion that happens after the Drain Resistance Test. Ergo, it's damage that is inflicted when there was technically never a Damage Reduction Test.
Frankly, I don't see the issue with letting Toughness trigger on Drain rolls, though. It seems "right" that being tough lets you sustain drain better than someone who's not tough. Plus, it's not all that abusive since if you're suffering drain, you must be a magician. And magicians have way more abusive things they can do with 12 karma than buy Toughness.