Thats a weird distinction, surely once you've included the Planes in your definition of 'Nature', then Nature includes Spirits? Especially as the conjure spell outright stats that Druids are conjuring nature/fey/elemental spirits
What are Shaman doing differently?
Well, at least in my personal understanding of it, Druids deal with the
essences in nature: raw elemental forces, beasts and their characteristics, weather, that sort of thing.
Shaman deal with the
consciousnesses in nature. "All that is, is alive." Everything has some kind of consciousness in it, and many things which have a consciousness here in the living world have, or produce, an associated consciousness in the Hereafter as well, the "spirit world". But that "spirit world" isn't actually separate from the regular world, it's more like a different way of seeing, a different wavelength of sorts.
Druids don't woo. They don't court, barter, or negotiate with the powers they interact with. They control said powers--sometimes to nurture, sometimes to vent wrath, sometimes to protect, but in all ways, those powers simply exist and are there to be wielded for whatever purpose the Druid intends.
Shaman
have to woo, court, barter, negotiate. They are inherently straddling the line between two worlds, the fulcrum point. That gives them influence, but it also subjects them
to influence.
That, to me, is an interesting distinction and worth exploring.