FAQ from PeopleforBikes.org.
"I have read the federal definition of an e-bike and it says that the top speed is 20MPH. How are class 3 e-bikes legal given the federal definition? The federal definition uses very specific language to delineate the top speed of e-bikes. The 20 MPH threshold applies when the e-bike is being
operated “solely” under motor power. However, e-bikes are most commonly ridden under a combination of human and motor power. The federal definition does not provide a top speed for when an e-bike is being operated under combined human and motor power. The class 3 definition clarifies this ambiguity by specifying the maximum
assisted speed for e-bikes at 28MPH. "
https://peopleforbikes.org/our-work/e-bikes/policies-and-laws/
Another FAQ:
"Why is this legislation needed? In many states, electric bicycles lack a specific vehicle classification. In these states it is unclear how they are regulated, or they may be interpreted to fall within terms primarily aimed at combustion engine vehicles such as mopeds or scooters. These classifications that were never intended to apply e-bikes. This legal scheme creates significant confusion for consumers and retailers, and hinders the electric bicycle market.
In order to clarify state law, and properly regulate electric bicycles like traditional bicycles, it is critical to understand the existing legal rules that govern electric bicycles. What other states use the classification system in this bill?
As of 2019, 18 states (Arizona, Arkansas, California, Colorado, Connecticut, Georgia, Idaho,
Illinois, Indiana, Maryland, Michigan, Ohio, Oklahoma, South Dakota, Tennessee, Utah, Washington and Wyoming) have passed laws that define three classes of e-bikes in their traffic statutes. Is similar legislation being advanced elsewhere in 2019? Yes. In 2019, identical legislation is pending in Massachusetts, Maine, New Hampshire, New York, Texas and Wisconsin."
(To date, its actually 22 states now that have adopted this 3 Class law.)