Given that this question has failed to attract an answer, I will speculate a little.
The (probably) false reasons
The only text I could find online regarding Freewheels and eBikes is as follows:
Most of our rear hub motors have a threaded side cover which takes a standard screw-on freewheel gear cluster, rather than the more modern cassette freehub system. While you can purchase freewheels from most bicycle stores, they are rarely available in more than 7 speeds and even then almost never with an 11 tooth small gear, which is essential to maintain a decent pedal cadence on fast ebike systems.
This implies that there is nothing mechanically special about freewheels sold for eBikes. They do however have unusuall many speeds (whether or not that is wise is probably another question) and a large ish range.
The large range is useful because it means you can do without a front derailleur.
Mind you, none of the above is special to eBikes. Road and mountain bikes also benefit from being able to go fast, and go up steep hills.
The (I think) real reason
Freewheels with 8 or 9 speeds are essentially useless on most bikes since they tend to lead to bent and broken axles. Therefore freewheels with more than 7 speeds are restricted to rear hubs with unusually robust axles.
Rear hub motors for eBikes typically seem to have slightly larger diameter axles. The axles on my bike is about 12mm. This is necessary to pass out the electrical connection through the hollow axle. This means it is possible to mount a freewheel with more speeds with less risk of bending or breaking axles.
It's worth mentioning again that the above is more speculation than anything else.