I don't know about Seattle, but here in Europe, when speaking of "e-bikes" people speak of a specific category that only goes up to 25 kmph (16 mph). Basically,

I don't know about Seattle, but here in Europe, when speaking of "e-bikes" people speak of a specific category that only goes up to 25 kmph (16 mph). Basically, it only goes faster than a normal bike if it's either very loaded or up a hill. E-bikes that go faster than that are, as you suggest, treated like light motorcycles, and require all the same things: license, insurance, registration, and equipment (motorcycle helmet and gloves in the EU) and are explicitly forbidden from riding on bike lanes.