They're not MANDATING a warning shot - they're going to stop the harsh prosecution of folks who opt to fire one. Currently, if you shoot someone, and it was self-defense, you're good to go. If you fire a warning shot, your looking at 10-20 years (mandatory sentences) depending on the situation.
Situational awareness is an on-the-ground responsibility and there are some cases where a warning shot might be useful and/or save a life from being unnecessarily snuffed out. I fail to see the downside of removing the out-sized punishments and allowing folks to make personal observations and choices.
Why waste a waste a round? Watch some idiot now come up with you now have to count to 10 before the next round can be fired.