"Training" here also doesn't imply some 21 year-old flight school student learning to fly. The aviators assigned to that unit are typically more senior people who've already done a tour or two with more conventional units.
I've lost count of how many times I've heard your sentiment already today, and I'm distraught by the general public's apparent lack of understanding about how things work.
If the system can’t be safely designed with a safe margin of error then it is too dangerous.
There are alternative ways and locations to train.
If there’s a nat sec issue have the planes hold.
Whether the system (i.e. separating rotary-wing and fixed-wing traffic there) can be more safely designed is a question for the FAA. The military aircraft are simply abiding by FAA rules for that airspace. Many more civilian helicopters are doing the same thing.