The name says it all. Firing guns at the sky is a traditional way of celebrating New Year's Eve and some other holidays in Latin American culture.
It use to be that at least one or two people were hit by falling bullets in greater Miami during those celebrations. I haven't been down there for many years, but at one time the Miami TV stations advised people who were outdoors in Miami to take shelter under bridges or in multi-story buildings with for a few minutes before and after midnight on NY's eve.
I knew it was common in Latin American countries, but never realized it was actually done, much less allowed in our country. That would scare the crap right out of me. I really am speechless.