All the native American bows I've seen were very simple affairs. Roughly made, but certainly effective. Take a stick, tie on a string, make it bend, make an arrow, kill a deer. Job done. No offense to anyone, but perhaps it was a matter of not knowing, or more likely not caring. I doubt "perfect' tiller was much of a concern. As long as the bow held up, spit out some arrows, and was strong enough to kill, job done. I guess what I'm really trying to say is, they probably didn't know better.