Who cares WHY?
That it was done is enough.
All any American needs to know is one individual was armed, and one wasn't. Why this happened is because this government floods the west with foreign nationals, many of which hate America before they've ever stepped foot on our soil. They don't fit in, they don't belong. All the bull shit about diversity is Communist lies.
Just like these government sponsored open borders, which have left a massive wake of tens of thousands of American victims in it's path for 40+ years...Thousands raped, robbed, shot, murdered, ran over by illegal drunks, burglarized et al...