Why do the majority of people on an ebike ride at a fairly low cadence? Around the place where I live the electric bicycle is gaining popularity and it's easy to pick those on such bike from far away not just from posture (rather stiff upper body even on steep hills) but also from their cadence: for some reason the majority (I'd estimate more than 90%) of the people I see on an ebike ride at a rather low cadence. It's hard to estimate how low but I'd say something like 30rpm; in any case: lower than any other cyclist on a normal bicycle no matter what type. So I've been wondering why and I cannot really figure it out (nor if they all do it for the same single reason). I don't really have the guts to just ride next to one and ask, also because I'm afraid to get frowning faces and answers like 'no idea'.

I was thinking it is maybe just as simple as 'because they can'. Maybe they all just think 'if it works ok spinning slowly, why spin faster'. But to me this is counterintuitive as even when I don't have to output a lot of power I'd still go for a decent cadence rather than a lower one as it just seems to work better and science seems to agree with me. And in any case I wouldn't go as low as they do. So I was thinking maybe they all were told to ride like that in the shop and/or there is some technical reason related to the whole drive system? Like it is more beneficial in terms of power usage or so? Or the bikes are just built with less gears and favouring the higher ratios?
There are (at least) two reasons. First, most (but not all) E-bikes use a control system that multiplies the amount of force or torque that you put into the pedals or cranks. Since power is the product of pedal speed and pedal force (or torque), increasing the torque allows the rider to reduce the pedal speed -- that is, the rider's cadence. Electric motors (can) produce full torque at any rpm, so these controllers typically modulate the torque output rather than the rpm.  

Second, if a rider on an E-bike were riding at "normal" cadence for his or her speed you would be less likely to notice the rider, so your observational sample is likely to be more heavily weighted to those with lower than normal cadence.