Pitchers go out of their way to mangle the balls in specific ways before they throw them, so even the ones that last a few tosses are in rough shape. You'll often see a pitcher simply throw a ball back to the catcher to have it replaced.

The math, by the way, is something like 10k fans @ $50/seat vs. 120 baseballs @ $0.75 wholesale. If there's any chance whatsoever that giving one to a kid in the stands will make him happy enough to come back for another game at any point in his life, it's well worth it.