> if there is demand for this kind of short rentals, why wouldn't they be allowed?
There is demand for a lot of things in a lot of areas, but it isn't allowed because it would be for the benefit of a select few at the cost of the many. Casinos, bars, hotels -- any kind of business, really, aren't allowed everywhere even though there may be demand for those services from people who don't actually have to live in that area.