IANAL, but I'd expect that the legal definition of the vehicle should be used by the insurance company - if an electric-assisted bike is a bicycle by definition (i.e. not a motor vehicle), it should be insured as a bicycle with just a rider on the regular home insurance (which is what I had on my bikes over the years). If it's legally a motor vehicle (i.e. with too much HP/displacement/whatever for a bicycle), it should be insured as such.
Unfortunately, insurance companies tend to rate things according to ripping people off rather than providing proper insurance at a reasonable price (and according to the real risk). Don't get me started though - I despise insurance companies for lots of old reasons...