Friends -- as a lawyer, I need to add my 2 cents on this....
First, as many people know, there's a big difference between CIVIL and CRIMINAL laws.
If you violate a criminal law, you have committed a CRIME. In that case, you can be arrested, charged, and prosecuted. Criminal laws are enforced by the police which means local city/town police, county sheriff, or the FBI in cases of federal crimes.
On the other hand, CIVIL law (also known as "tort law") is not enforced by the police -- it's enforced by YOU, either by filing a civil lawsuit in court, or by hiring a lawyer to bring the case on your behalf. Violating civil law is not a crime; it's generally a private civil matter that simply means anyone who is hurt/damaged can bring a lawsuit seeking to recover money. In certain cases, you can also seek an injunction prohibiting certain conduct from happening again in the future.
So, when you are asking whether it's OK to shoot down a drone, you are really asking TWO different questions:
1.) Is there a CRIMINAL law that says this is illegal?
AND
2.) Is the shooter subject to potential CIVIL liability?
A couple other points to consider:
Forget about criminal laws for a minute because the CIVIL side of the issue is 100% clear -- if someone else damages your property, you CAN ALWAYS sue them and you CAN recover money damages for the value of your property. It doesn't matter if the "property" is a car, TV, cellphone, or Mavic -- if someone else damages your personal property, that is always "unlawful" in the sense that you can sue them for damages.
Also, the fact that you are flying over someone else's property is almost certainly NOT a defense in a civil case. If someone accidentally parks their car in your driveway, this might be a form of trespass (same as flying over their property), but does that mean you can shoot the trespasser's car? ABSOLUTELY NOT. If you shoot their car, you will be liable for damages. Same thing with shooting down a flying drone.
Bottom line -- unless a state passes a specific individual law that says it is OK to shoot down a drone that's flying over your land (which I think some states are considering), then trust me -- shooting down a drone for any reason WILL expose you to civil liability 100% of the time -- PERIOD.
Now what about criminal law? This is a very different issue. For one thing, criminal law violations general don't require any damages -- if you break a criminal law, you can be arrested, even if no one was hurt (this is NOT true in a civil case -- if someone shoots at your drone and they MISS, you can't sue them because they have not actually caused you any damage).
Also, even if there is no specific law banning shooting at drones, any person who does this could still face potential criminal liability for things such as unlawfully discharging a firearm, reckless endangerment, disorderly conduct, etc. Trust me -- it is not necessary to have laws banning every little thing. We have so many existing criminal laws that apply to a broad array of conduct, it's nearly impossible to say: "Well, there isn't a law that applies to this situation, so I guess we need a new one."
Final suggestions:
1.) If you have a gun and see a low-flying drone, DO NOT shoot at it.
2.) If you have a drone and someone shoots at it, call the police.
3.) If you really want to fantasize about shooting down drones, watch this (the guy with the beard happens to be suing a client of mine):