Hi John, an explanation of the laws:
The fundamental has always been that with untested, unlicenced and untraceable riders, in the interests of public safety, e-bikes must remain bicycles.
In the UK, the original 1983 EAPC law restricted e-bikes to 200 watts assist and 12 mph assist speed, but that speed was later amended to 15 mph to more closely match the EU 25 kph limit, but accompanied by a UK lower age limit of 14 years. The 200 watts was considered to be a match for what a rider could input, and that is in fact the average for a fit person over more than an hour duration. Throttle control was allowed by default since there was no mention of how power should be controlled.
The EU view was that the power should be 250 watts with a 25 kph limit, but to keep them as close to being bicycles as possible, the power application should be through the pedals just as it is on normal bikes. A further EU ruling is that the power should phase down as it approaches 25 kph, but the degree and steepness of the phase down is left to the designer. The EU sensibly has no lower age limit.
In both cases the assist speed is that considered to be the upper end of utility cycling and it's not intended to match the sport riding speeds of 20 mph or more, on the basis that sport riders don't need assistance. The weight limits are mainly to restrict the mass that could impact on anyone in a collision, there being a big difference between the hard mass of a vehicle and the soft mass of someone's body. There is a factor of control where weight is concerned, riders being untested for competence.
Similar conclusions have been reached elsewhere, the Japanese have 25kph and 250 watts, but their power phase down is specified from a very early stage equal to 9.4 mph and it's effect is quite severe. In China the assist speed limit is 12 mph but they are generous with power at 700 watts. You'll gather they don't like pedalling, even uphill.
Currently the UK law is under review and will be replaced at a future date, targeted for 2016, and will then more closely match the EU regulations. No decision has been made yet on retention of throttles, but the DfT has said that existing e-bikes will not be affected by any change.
Overall it's easy to find fault with the laws, but it should be remembered that anyone untested and untraceable can legally ride an e-bike no matter how incompetent physically or mentally, the only powered vehicle without any such restriction. Therefore for public safety reasons, the bar should be set low.