A bit of history is necessary to understand that.

Class 1 and 2 come from preexisting federal definitions that a handful of states had adopted. Class 1 is no throttle, class 2 allows a throttle.

Class 3 however, actually comes from the European s-pedelec definition. It never included a throttle, because those speeds with a throttle were covered under a separate moped definition instead. The regulations for it were imported almost verbatim, because the companies backing lobbying efforts to make e-bikes legal were mostly European at the time, and that made it easier for them to import their fastest bikes - simply set the display to MPH and done. The cheap Amazon bikes hadn't really entered the picture yet.

In retrospect, now that we've seen all the mopeds and motorcycles posing as e-bikes and the lawless attitudes of those riding them, it was a good decision to exclude a throtte from class 3. By imposing a forced choice between a faster e-bike that has to be pedaled, and a slower e-bike that allows for a throttle, it reduces the appeal to those that aren't responsible enough for higher speeds, while still meeting mobility needs of people who can't pedal, and of people trying to build up the endurance to pedal longer.

Since anything observed at any previous moment to be going over 20mph can now attract additional scrutiny from cops, and finding a throttle on a bike that cops have previously observed to go faster is grounds to seize the bike, that puts pressure on riders of unlocked bikes to take it easy and stay off the radar.

In short, it started as a regulatory quirk, but became a trap for people who want to break the law and ride motorcycles around without a license. And that's why we can't have nice things. Pretty much the same reason mopeds now require some sort of license in most states - too many people acting like they were above the law ruined it for everyone.