I have never been told not to use warning shots, quite the opposite. From the academy to the streets, the protocol has always been the same. Use warning shots only in extremely limited circumstances.
If I choose to do so, aim the weapon at something that will ABSORB the bullet, such as a tree trunk or packed dirt so there is no ricochet. Never aim the weapon into the air as a warning shot. Document the use to the nth degree.
The escaping felon was not a scenario given to us as one where a warning shot was appropriate. The scenario used was if you were surrounded or another officer was surrounded and you had to get to him and the crowd can't hear you or won't hear you.
Now, I won't debate the logic of it, but it was what was taught.
Guns! More guns! I need more guns!