I'd assume decades of anti-gun propaganda.

As a gun owner and sometimes instructor who's not a drone hobbyist, I don't like this because it's very hard for it to avoid violations of Rules 2-4 (3 unless the fire signal as well as button are very very well protected, e.g. encryption):

1. All guns are always loaded. Even if they are not, treat them as if they are.

2. Never let the muzzle cover anything you are not willing to destroy. (For those who insist that this particular gun is unloaded, see Rule 1.)

3. Keep your finger off the trigger till your sights are on the target. This is the Golden Rule. Its violation is directly responsible for about 60 percent of inadvertent discharges.

4. Identify your target, and what is behind it. Never shoot at anything that you have not positively identified.

Hmmm, and modern handguns are designed and built to not unintentionally fire if dropped in normal circumstances, i.e. from hand to concrete floor. A drone could subject a gun to greater stress.

There's probably a fear of violating an FAA rule (if there is one, and there likely is) of a civilian arming a flying vehicle.