It's the fundamental difference between Alien and Predator. An alien would never team up with a human because aliens are mindless animals. Predators have intelligence, and some degree of honour.
I didn't think the elder giving Lex a spear was very appropriate, though. Harrigan killed a predator, and all he got was an old pistol the predators stole on one of their hunts (which seemed to be reluctantly given). I think Lex was spoiled a little when she received that amount of respect from the predators.