On most ebikes without a torque sensor as long as you turn the pedals a little the bike will sit at its maximum speed of around 15mph anyway.
That's perfectly legal.
l don't understand why that is any different to having a throttle:
you can ride slowly with a throttle, like when negotiating a gate or when the cycle lane is full of riders. When you ride slowly, the throttle replaces the torque sensor: instantaneous response and controlled acceleration.
With the pedelec sensor alone, it depends very much on how the controller is programmed: current control or speed control.
If it's programmed for current control, then the acceleration is moderated by the setting of the assist levels. If it's speed control, each assist level has a speed setting. The motor will give a bit of a surge every time the motor starts up then the power reduces as your speed gets nearer the pre-programmed speed. Current control is good for going slow because the motor does not give a big surge.
Either way (current control or speed control), the controller would be programmed such that your speed is still limited by your pedalling. You can still control your speed by pedalling slowly but it's rather unpleasant compared to a throttle or a torque sensor.
The thing is every person has his or her own preference. For most people, speed control is fine. It's pretty much what you would want the bike to behave most of the time: the bike accelerates quickly at startup and forces you pedal a bit more when you go faster. For more long term e-riders, current control makes the bike feel more like a push bike.
Bafang made available a piece of software that lets the user program the assist levels individually on their BBS kits. That's an advantage, you can pretty much get the motor to behave the way you want although it may void your warranty.