Because he is? He is the reincarnation of mankind's ancient and most powerful shamans. Hence shamans by association are spiritual folk, such as many Natives all over the world are, not just endemic to Native AMERICANS alone.
I understand the need for drama and plot to the 40k universe, but when your trusted "son's" choice of armor involves deamon eyes on his power-suite, along with an evil-looking power claw, one can't help but ask the question, "How did the All Father NOT see this coming!?!".
Before the Heresy that eye was known as the Eye of Terra. Also the Warmaster's power claws were crafted by the Adeptus Mechanicus overseen by the Imperium. Chaos likes to twist things like this, making the Eye of Terra the Eye of Terror.
Nah, I made some research, so the Eye of Terra = Eye of Horus The staring Eye banner is the personal banner of the Warmaster himself. Beneath the staring eye is a decree embroidered in golden thread: 'I am the Emperor's Vigilance and the Eye of Terra.' I made a mistake on Sanguinius' armor, those symbols are not eyes but blood drops, symbold of the Blood Angles. My bad.
I guess that wouldn't be grimdark enough. Also, at the time of the Great Crusade Terra has already lost all its oceans and wildlife. The people of Terra were living inside enormous hive cities and underground. Still, I don't know why the Eye was red and black and I couldn't find the story behind it. Sanguinius' armor also had eyes like these, however he remained loyal to the Emperor.