> why is the throttle the issue and not just the 45MPH?

The bikes with throttles are not legally e-bikes, so the products on the market ignore all of the other e-bike restrictions too. They have much more power and higher top speeds.

Even if they were fully limited, pedaling ensure more rider engagement and changes how people ride them. When you have to put some effort, however small, into moving the bike around you ride differently than if it's an effortless throttle input.