Because the P-51 must come within range of a wirblewind in order to accurately drop bombs. That way, the vehicles aren't reliant on a lucky intercept.
I really don't like it when aircraft can strike with impunity, and the GV's must rely on other aircraft for ANY sort of defense.
Honestly, I'd rather take my 7.92mm against a P-51 as opposed to some unknown aircraft flown by a pilot of unknown skill at attacking bombers flying against a Do 217 (a fast, reasonably well armed bomber), where an intercept isn't guaranteed.
Why? Because I've killed attackers with the 7.92, because its a known quantity vs an unknown, and above all, because its me manning the gun.