Also no offense, but going 20 mph in a bike lane or worse a sidewalk would be very bad. An ebike is not a bike, but a moped under the classic definition, a bicycle with pedals and a engine to provide supplemental power. It's just not a gas moped.
Also... you sound like you've never ridden a road bike seriously. 20mph in a bike lane, especially while going downhill, is pretty meagre for a standard road bike. No assist needed.
Hell, I'm fat (ish) and I can hit 25 on flat ground on my road bike with some effort (although staying there is draining). I can easily do 30 going downhill.
I'm regularly passed by folks doing more than 30 on hills where I live.
How? 20mph for a road bike is slower than what trained cyclists do
Bike lanes are hit or miss. I'd ride over 20 in a protected lane, but not next to a line of parked cars (door zone).
Step 1: Peaceful citizen buys an ebike to make commute easier. (person is elderly, permanent injury, has steep climb daily, other reasons that are their own)
Step 2: Rider Wears helmet, slows on oncoming traffic, stops at lights, and is generally a perfectly fine rider like many bike riders
Step 3: Concerned mom-type notices faster bikes with engines in lane and complains
Step 4: Law passed, Rider is now doing something illegal
Step 5: Ticket/Jail/Confiscated bike despite no evidence or numbers that rider was a danger to himself or others any more than cyclists
This is literally every law that affects peaceful citizens.