I got Shat on my head

I think the reason for that is because the Naaru need all the help they can get to defeat the Burning Legion. Seeing the Horde as allies in this crusade, they may have accepted help from the Horde or offered them. Here's a more detailed explanation..

Orcs: They're in Outland mainly to find out more about their ancestors and they also continuously battle the Burning Legion.

Trolls: They're closely allied with the Orcs so they'll most likely tag along.

Tauren: They value life and despised the Burning Legion greatly. They also honour Thrall and his companions and the Cenarion Circle so the Naaru might have seen good intentions from them.

Undead: They still battle the Burning Legion although their true intentions are still a mystery.

Blood Elves: The Blood Elves in Shattrath are actually defectors from Ka'elthas Sunstrider's army. They are led by a certain Blood Elf, they marched to the Shattrath's gates and offered their assistance to the Naaru.

How's that?:)