Okay, so. The Werewolves kill Father Wolf, right? And Luna curses them for it, right? But they're like "We'll totally take over his duties!" so she kinda lifts the curse, and blesses them. But the curse remains on the Pure, the guys who weren't in on killing Father Wolf, and are in fact on a Roaring Rampage of Revenge exactly for that kill? What did I miss? Is Luna just kind of a dick?
Luna lifted the curse of silver from the Forsaken because they accepted responsibility for their actions. The Pure, however, didn't do anything to stop the Forsaken from killing Father Wolf and made no attempt to continue his duties following his death, instead basically going along with whatever more powerful spirits tell them to do. It's suggested that Luna would be entirely willing to lift the Pure's painful aversion to silver if they'd actually ask for it, but they're too caught up in their idea that Bitch Luna engineered the downfall of Urfarah or is otherwise evil to seek forgiveness — the curse was on all of the Uratha in the first place and the Pure turned their backs on Luna long ago. Besides this, the Pure are composed of werewolves without auspices. Barring the use of a special rite practiced solely by the Pure, having an auspice is the natural state of every new werewolf and the loss of one is done through another rite (again, performed only by the Pure) by carving or burning out their connection to Luna
That's just the Forsaken's account of the story. The Pure view it quite differently, to say the least. According to their tribal lore, Father Wolf's murder was Luna's suggestion in the first place. When the hour came, Rabid Wolf and Silver Wolf did rise up to defend Father Wolf (not Dire Wolf though, that guy was kind of a dick), but they were outnumbered and helpless to stop the deed. And yet Luna still cursed them. Not out of revenge, but to weaken their kin, making them easier to manipulate. The Pure also don't view Auspices as a natural aspect of the Uratha condition. After all, werewolves didn't have Auspices back in the golden days of Pangaea. Father Wolf himself had no Auspice. The Auspices are entirely Luna's doing, and to view it as part of your inner nature is amount to heresy.
Because Luna is largely unseen, no werewolf alive today was present for the events in dispute, and the spirits who could say are notoriously mutable in their nature and their minds (and therefore their memory). The particulars boil down to theology and mythology anyway.
Depending on story teller, venue used, or even (gasp) Book utilized, the stories vary to the point that in Skin Changers (yes go read it) and Changing Breeds (as much as you might hate it) Silver hurts ANYTHING that is connected to the spirit world...why? Old Folk Tales once stated silver is like liquid light (The Other Wiki can probably shed some light on that whole Real Life mythos) and is thusly holy (many religions revere it after all as it is white as light and once refined more pure then gold) so Clap Your Hands If You Believe factor here and POOF Silver + Faith + Unknown Numenia constantly in effect = Shifter Poison.
One of Luna's epithets is Queen of the Shadow. Her portfolio includes, among several other things, shapeshifting. From a metaphysical standpoint, every shapeshifter is a child of Luna. Luna didn't just curse the Uratha with the bane of silver — she cursed all her children. Think about it.
I heard about a supplement about back before the Forsaken killed Father Wolf. The Dark Age, I think it was called. I can't confirm anything, but I think it turned out that the silver weakness wasn't a deliberate curse, it's just part of their nature. Like passing on a genetic disorder to your kids. Luna just made it worse for the Pure, but they already had it.