FPV quadcopters are legal in the USA, but they can be used illegally. The thing to remember is that no matter what you are piloting, or controlling, It is the operator that is held responsible for being safe. If you fly your quad into a person, you will be held accountable, if you fly your quad into air traffic, you will be held accountable. If you use your quad to spy on people, You WILL BE HELD ACCOUNTABLE!
I like to equate it to gun ownership (a little extreme… I KNOW!)
Guns can be used for sport, or for killing. It is in the hands of the operator to decide.
Quadcopters should be fun, but they are not toys…. They are serious machines that can cause major damage to personal property, and therefore should be treated with respect.
As far as the law in the United States of America is concerned, There is much debate about the legality of these machines. The current model aircraft laws state that you have to stay under 400′ altitude, and that you must maintain line of sight. FPV allows pilots to violate these laws, so it is advised that pilots have a spotter with them to keep the aircraft within line of sight and help the pilot.
Keeping the craft below 400′ is very important. It is also important to make sure to stay away from airports.
It is illegal to fly in national parks. A great site to understand where you can fly and where you can’t is Don’t Fly Drones Here!
As far as the FCC goes, It is illegal to use certain frequencies and certain frequency power without a license. Most FPV Quadcopter FPV gear operates at 2.4 GHz for Control, and 5.8 GHz for Video. These are wifi signals, however the max allowable power output is still governed by the FCC.
According to the FCC 15.249…
Fundamental frequency Field strength of fundamental (millivolts/meter)
Field strength of harmonics (microvolts/meter)
902-928 MHz 50 500
2400-2483.5 MHz 50 500
5725-5875 MHz 50 500
24.0-24.25 GHz 250 2500
Some Quads that run in the 400MHZ, 900MHZ & 1.2-1.3Ghz range require a Ham license in the united states to operate.
As for other countries, I have no idea what the rules are, but I know that there are specific frequencies and powers that are allowed, and it is important to check with local laws before operating FPV Quads.
If you have no idea what I am talking about, you should have a look at the Electromagnetic Spectrum.