I am not sure how this could be enforced other than to say that if you are involved in an off-duty shooting mishap, and did not complete this training, you could be found "not in the line of duty" when it comes to any medical treatment or death benefits.
FYI, the only Death Benefit that is tied to the LOD is the Survivor Benefit Plan. You pretty much have to be in the commission of a felony at the time of death to be determined NOT in the line of duty. Even if you are committing (or just finished committing) said felony, all death benefits are still paid. You just don't get military funeral honors.