There is no forced cadence. The motor is not fixed at a particular RPM; if the motor were unloaded, it would spin a lot faster than it does. It supplies a certain amount of power, and the bike hits a certain ground speed, and through the gear ratio, that determines the motor's speed.
If you only want to go that fast, then you don't have to crank the pedals, and it's awkward. However, you can crank the pedals to go even faster.
That's the point of using the motor. You + motor is faster than you alone or motor alone. Or: you + motor requires a lot less effort on your part to maintain a certain speed than you alone. Suppose that you can go 30 mph, but with great difficulty and not for very long. Suppose that a given motor by itself can go 20 mph. If you use that motor and your own muscles together, you can go 30 mph with a lot less effort, and sustain it a lot longer. The cranking of your pedals and cadence will be natural; you're cranking 50% faster than what the motor can do by itself. It will be like cycling in the slip-stream of a truck, or down a slight downgrade. To the spectators, you will just look like a strong cyclist.
About the 100W, almost any amount of cheat torque can make the difference between placing N-th and N+1st. Every second counts.
Even a 2.5W power boost could be a game changer. If your muscles put out 250W, 2.5W is 1% more, which is significant. It's about 36 seconds shaved off a one hour haul. The game has changed from a neck-and-neck race to the front runner having a 36 second lead.
A tiny, light-weight motor of just a few watts that would be useless in a commuting bike used by someone who is out of shape and cannot climb hills could nevertheless win trophies for a cheater.
One minute you're racing just beyond your lactate threshold. Flip a concealed switch, and a tiny power boost drops you within your threshold.