Given the conditions on Pandora, once you have become an adult, you never go anywhere without a weapon to defend yourself. Indians did the same thing, it's self protection against nature not each other.
I found it irritating also that Jake's knife is thrown down in Hometree, he gets it back after he wakes up and loses it then finds it again.
Also..someone had to find Tsu'tey's body and retrieve the clanhead neckpiece for Jake to wear.
As for the knife position, it could be personal preference..where did Tsu'tey wear his? I think it was in front. I don't think Jake had his in front because of his previous experience..in the scene where he's carving the stick to a point... and later when Tsu'tey trips him with the bolas, I think he pulls his knife from his belt. (Have to watch it for the umpteenth time to see that.)

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