Legislate one, but not the other? E-bikes are currently as legislated, as they should be, the guidelines are just considerably simpler than some other vehicular guidelines are. I don't wish to create a separate class, so much as make sure that these vehicles are properly placed in their correct categories. Forgive me, but I don't believe that 165lb scooters are correctly labeled as bicycles.
I've 2 formal, regulated suggestions, and one informal suggestion. One might say that this task is insurmountable, but government transportation agencies have long practiced the art of classification.
The simplest way would be to begin with requiring the manufacturer to provide proof that the pedals and drivetrain are not only functional but fully adequate for use in a consistent application. They'd need to have some sort of semi-permanent attachment system, like as mentioned - those featured on some folding bicycles.
Secondly, the weight limit needs to be revised and based upon the abilities of an "average" test group - no gathering up 30 seasoned bike tourers, i'm talking about pulling a pool of random subjects. Figure out what the typical reasonable weight limit is to take a bicycle through an obstacle course designed to test rider ability, and work with that number. I'm almost certain that a reasonable number for a typical 2 wheeled bicycle would be *nowhere* near the current limit of 165lbs at a low speed. There's a far cry between riding a 165lb motor vehicle and pedaling one, and no electric vehicle should be allowed to circumvent that distinction. Add a second classification for vehicles with 3 or more wheels, with the same electric restrictions and requirements, but a different distinct and tested weight limit.
Thirdly, quality control needs to be done. From what I understand, many of these things cost many times what a basic electric kit does, but provide poor value in comparison. While this is something that is difficult to enforce officially, in a sort of way its up to those who already have the experience in these products to help guide people - weed out bad ones, promote good ones. This is a relatively new field for North America, and unfortunately quality is something where minimal quality is often a disappointment for new buyers. Help people out with making good choices!