Some airbnb hosts only rent their property while they are present (airbnb “private room” listings). Others only rent when they are absent (airbnb “entire place” listings). But some hosts who normally rent “private rooms” while they are at home may also want to go away for a weekend or a week, and rent out their place while they are on vacation. The ability for them to do so is very important to airbnb.
If a property is in fact the primary residence of the host (where they live most of the year) going away for a short-period of time does not reduce the stock or rental housing. However, it can still cause problems for their neighbors.
If periodic host-absent “entire place” rentals are permitted, it may be easiest to mange this by limiting host-absent “entire place” rentals to a maximum of 30 days a year with advance notification of contiguous neighbors. This keeps housing units from being turned into de facto Vacation Rentals and makes sure neighbors know what is going on.