Define Dangerous. After all, some of these brands carry CE markings and the like. Some have fuse and manual breaker switches (but probably not most of the cheapies). Are skateboards also banned? Looking at crime numbers people being king hit with fists is more likely than being donged with a skateboard or umbrella.
So they are dangerous, even though not one has ever caught fire - sort of like iphones through the post, and all the gazillions of laptops taken aboard. - no evidence behind the assumption. Plus all batteries have internal thermal fuses - well should have anyway. Sort of like umbrellas - they could be used to rob and stab and dangerous in the cabin. I am not sure how many people have been attacked by umbrellas. And I am pretty certain electric wheelchairs go into the hold. And probably have a bigger battery. I suppose, like guns, the gun people could allow 'safed' hoverboards aboard.
Breaking down outrageous fears, a bigger battery is not needed. Bigger batteries are harder to flush down the toilet. Those scatter scanners should be able to tell the difference between wet acid cell and lithium.
I suspect the real reason is some MIGHT be used to conceal something bad - but I do not know the x ray profile. But water is already identifiable = yet water is you know, dangerous too.
More likely airports don't want kids doing wheelies or zooming at top speed to the gates, or seeing pax get off and hovering past the rip-off transport area, and catching public transport off the airport premises. I can imagine a drunk pax crashing into the duty free area. In fact business people zooming past the duty free shops not buying anything would be a financial danger.
Airlines don't want lots of 6kg or more things sucking down fuel and increasing takeoff weight. Airports don't want tricky things to slow down security theatre- strollers are bad enough.
Just to spoil patents, if hover boards had crude altitude/pressure switches, that could be a fourth protection. I look forward to someone getting airline approved hover boards certified.