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.)