Then Assassin, Peace Cleric, and Conquest Paladin should, by that same logic, have alignment restriction?
And arent ''Dark magic'' in DL just certain restrictions on spell schools in the past setting books?
And the gods are said to be gone/distant, so I dont think worshipping is the right word here. Following the precept of ''black magic'' under a the auspice of a magical moon, maybe, but there nothing religious/sacred in their following. Just look at how they switch between the color of their robes. They are closer to political/power factions in-game than cults. Like the different color of Jedi, really.
But its been a while since I read the book. Yet, Raistlin did not strike me a disciple of a dark god, more of dude with an agenda and love of dark magic.