It doesn't really matter whether the property you are letting is your primary residence or not, really. What matters is that you aren't using it.
AirBnB caters to shorter, less formal stays than a normal rental site. It also presents a much improved User experience. It also implies that you can get in on this game if you don't actually own the property, but whether or not you can legally do this is up to you to know, not AirBnb.
It doesn't really matter whether the property you are letting is your primary residence or not, really.
It matters in terms of security. If you're using the property, that either means that you have a considerable amount of personal information and valuables on-site, or that you'll have to take them off-site to secure them.
If the property is a dedicated vacation home, then this will generally not be the case, and coupled with deposits, property insurance, etc, the risk to the owner is significantly reduced.
Good luck getting property insurance on an apartment you sublet.
There have been online vacation property rental sites for years.
AirBNB, however, sells itself as a mechanism for making some extra cash on your own actual day-to-day residence.