There's a govenor. Soon as it sees 20MPH it simply cuts the power to the motor until it drops back below 20 MPH at which point it can assist again.

You have to try one to find out that 20 MPH isn't that slow. My "unlimited" version (I simply defeat the governor) only goes about 25MPH and I can tell you: it's fast. I'm always speeding past others. The difference is that you can sustain 20MPH for a long period of time with an eBike.