These are the product of a pact with a devil. the exact origin is unknown, but many different Infernis will tell you a differing origins tale. It's almost like their cosmologies are sundered.
Infernis are frowned upon, literally by humans and unlikely to ever be @ granted the rights and privileges of citizenship unless those rights are spoken of in ironic tones. Many of the religions practiced in the valley have myths and legends like Hades and Persephone which normalizes the stigma of being from Hell to a great degree. Let's just say opinions vary, about tieflings. Sure they are different, but they are understandable different and perhaps even messengers of the Underworld.