It's really just Game Freak logic. I suppose it's because they don't have Levitate or a Flying type, and nearly all the Pokemon eligible for Sky battles have either Levitate or are part flying type. I think if you become too concerned with "a Pokemon looks like it could fly/ float", you would end up including stuff like Mew and Jirachi, which can technically float through the air from their Psychic powers. I think just limiting it to Flying types & Levitate users would make categorisation and explanation easier, because saying "a Pokemon looks like it can Fly" is pretty much based on personal opinion.
Dratini is listed as eligible for Sky battles on Bulbapedia for some reason.