Now let us take air resistance into the account. It is true, on its way down, the projectile reaches what is called terminal velocity due to steadily increasing friction caused by +g. However, the terminal velocity of the projectile is dependent on the density of the projectile and the aerodynamic nature of its shape. High density coupled with good aerodynamics results in high terminal velocity. Bullets are usually made of lead which is a highly dense metal. Bullets fired from high powered rifles are quite aerodynamic and have high density. As a result they have high terminal velocity. It is not uncommon for bullets to fall down with a speed of 300mph if fired straight up. Now imagine a 7.62mm ammo falling on your head with a speed of 300mph. It can and will kill you. It is dangerous to shoot guns in the air.
It is ...
however, you haven't presented all relevant variables.
Let's take an urban area, which, in the U.S., has about 10,000 people per square mile.
1 square mile = 5,280 feet x 5,280 feet = 27,878,400 sq feet.
The average person may have a profile, when view from the bullet, 5 sq ft. Some people will be standing - they will have the smallest profiles - others will be laying down on their backs or face down - they will have the largest, but I'd bet on average is no more than 5 sq ft. So those 10,000 people in that square mile is 50,000 sq ft of people
50,000/27,878,400 = 0.001793503 = 0.1793503%.
This means if you fire a gun into the air of an average urban area in the U.S., the chances of the bullet actually hitting someone is roughly (rounding up) 0.2%, or 1 in 500.
This of course assumes the bullet doesn't get lodged in the roof or part of the building above the person it would otherwise hit. The presence of multiple floored buildings and/or buildings with thick and/or stronger roofing material (or just a lot of crap piled in the attic) would reduce the risks.
Its definitely not anything I would do for fun and anyone who does it should be punished - but its not the most horribly heinous thing you could possibly do.