In theory it was illegal to sell firearms to the Indians.
In theory it was also illegal to sell alcohol to the Indians.
In practice, both were largely disregarded by hundreds of traders who did very brisk business with many indian tribes in both commodities.
The simple matter was that, once a trader cleared "civilization," and got into Indian country, there were few, if any, effective policing mechanisms to stop him from selling anything he wanted to the indians.
And the theory that an Indian could fire 3 arrows a second?
Not even the best bowmen could do that.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.