Deliberate mistake: When King Willie is confronted by the Predator in the alley the Predator approaches him cloaked stepping through the puddle of water. You can see his footsteps causing the water to splash up. However, you can see the water splash up and go through his foot. The Predator is only cloaked, not intangible so the water should have splashed upon his foot, not go through. Water against his foot would be visible too, even though he is cloaked. (00:47:45)
Deliberate mistake: Whenever the Predator is in camouflage, it does not have its signature dreadlocks. This is confirmed in the DVD extras as being too difficult to include them when creating the visual effects.
Answer: As a reward for besting the Predator in combat, he proved himself a fierce warrior worthy of praise and the gun was a token of their respect for his strength.