Where I live (western EU) it's simple. If the speed is limited to 25 km/h it's an e-bike. Above it's a speed pedelec and requires registration plate + insurance.
Just to be clear - it's not the speed that is limited, it's the assistance that is. You can still pedal above 25km/h on your ebike just fine and it's legal, but it has to stop giving you assistance above it(in contrast to say, mopeds, which have to be speed limited to 45kph).
Most US states allow 28 mph e-bikes. Some limit it to 20 mph. E-bike laws -- like all traffic laws in the US -- are set by the states. (as you may notice, even our cars do not have "united states" license plates)
