Farnam refers to such phrases as "can't help you", "can I help you sir?", "don't move, drop your weapon", etc, as canned phrases that we learn to use automatically. It's done by practicing, just like we practice shooting.
I think that bad language is typically a result of our own fear and anger (the two are related), and it's already been pointed out that it's not good to appear to be out of control.
Same goes for calling 911---don't act like you've lost it.
Everything we do becomes a matter of public record, and "don't move, drop your weapon" spoken loudly so that all eyes are on you and Bubba, may be as much (I'm not saying more) for the benefit of the record (like witnesses statements) as it is a warning to your assailant.
Doubt we'll ever be more scrutinized than in the aftermath of the type of encounter we all hope never happens.