In reality there is no reason to prohibit the provision of a throttle on a pedelec as long as it does not override the crank sensor, it simply gives the rider the ability to reduce the amount of assist available to less than the rated motor power.
Ian.
Sorry, I'm coming in a bit late on this. What Ian describes, a throttle to let you reduce power input when pedalling, is useful in its own right, and is something I really wish I had on my Urban Mover 55. I don't always need full power, and if I could reduce the input, I'd get more exercise and the battery charge would last longer. But when UM is controlled by pedal sensor, it's giving you full blast, whether you want it or not.
Yes, it does also have a throttle, which over-rides the pedal sensor. That gives me the pull-away boost, which as noted elsewhere, is useful. But the throttle will only drive the bike up to 10 mph, so it's only giving me partial power and isn't practical for when I'm going along. The old Heinzmann-powered bike I used to have was completely throttle-controlled. I liked being able to put in as much effort as I felt like (usually to about 12 mph) then "top-up" with the motor, for hills and headwinds. With the UM, I can't contribute unless I get the speed up to >15mph, something I don't always want to do on congested city roads.
If pedelec-only ever becomes firm law in Britain, I could live with that, but I'd still like to control the power input, even when I'm turning the pedals. A useful modification to the law would be to allow throttle-only power up to 5 mph, to cover the pull-away situation.
Mary