There are pictures of natives shooting like that, there's a famous one of Ishi squatting down with the bow at about 45degrees.
Google Ishi if you've never heard of him (a well documented native american, he was the last of the Yana because his tribe was slaughtered by white men as they put telegraph poles across the US)
I'd stand the question on it's head... "Why on earth would you want to hold a bow absolutely vertical?"... unless of course you have a truck load of modern paraphenalia bolted to it?
Another point, stand relaxed with your eyes closed, think Yoga or Tai Chi, gently raise your arms out sideways, fingres and thumb lightly spread... relax, let them float... now don't move... open your eyes look at you left hand? Is the thumb at about 45dgrees or is it vertical? That would seem to me to be a natural angle to hold a bow.
So finally to answer the question, there is no reason to believe that canting the bow hasn't always been the way of shooting it, I don't expect early man stood up straight and tall with a vertical bow to try to shoot food, he'd likely scare everything in the vicinity.
Del