Balloons are cheaper
Many police forces use Glock 17 pistols, which have a standard capacity of 17 bullets[0]. Ignoring how fake the muzzle blast looks, the "officer" empties his 20 round clip into the air. I can't find a citation, but I believe police are not meant to fire unless they are in danger. While an officer could certainly ignore this, one would know that they could have disciplinary action take for emptying an entire clip haphazardly into the air in a busy location.