And the federal rules ONLY govern ebike manufacturing standards. The federal law being discussed here is not really a law. It comes from the Consumer Product Safety Commission, a branch of the Federal Trade Commission, which simply sets manufacturing safety standards, and governs manufacturers (via cease-and-desist letters when they choose to issue them).
The state statutes that exist in most of these United States are modeled after California legislation that initially established the 3-class system. It also carried over the federal '750w' limit.
Except there is no such limit. And most people who read it, legislators included, miss the boat on this.
The standard as-written actually reads 'less than 750w'. And all of the 3-class state legislation I have read also propagates this definition. So a 750w motor is non compliant. To be compliant with the ebike definition, the rule is actually 749w or less. Everyone ignores this, right? Not in Hawaii. The City of Honolulu revoked bicycle registrations there for 750w ebikes specifically because they were over the 749w limit.
Hooray for bureaucracy. Nothing rated for 750w is legal. Oh and also the federal standard has a caveat in it that establishes a speed limit, but the speed limit is dependent on the rider's weight. So a heavier rider can have more power than 749w to be able to hit the 20 mph limit. None of the states use this part of the federal manufacturing standard in their legislation.
Don't spend too much time worrying about any of this. The CPSC is well aware of the discrepancies in the standard and in how the states are implementing it. They've unofficially gone on record (via a reported conference call with CPSC attorneys and a quixotic busybody who was trying to demand they force the states to conform to the federal standard) as saying they want to leave this to the states.