Who is this "obvious" to? Clearly not you, since you think it's a horrible idea. Electronic skateboards have been around for quite awhile, and until now, they've been very heavy and mostly impractical. Yet, they are a viable product in the market and popular enough that people are making money selling them. One of the reasons this product is a leap forward is because it's lightweight enough to be used as a normal skateboard without electric power.
Simply stating that there is no place for this product on the sidewalk doesn't make it true, and I personally disagree with you. Stating that it is a "danger to everybody on the roadway" is absolutely absurd and simply untrue. Any vehicle that is under the control of a competent and observent operator will not be a danger to others in the least.
Here's the real problem: The contrarian, negative, hostile commenters on HN are the vocal majority in a lot of cases. Personally, I'm really, really tired of this even being accepted by the community.