Because the title is misleading. It isn't costing 0 USD. If something costs money, be up front about it. Because a lot of people do not have much money to spend freely, and by saying 0 USD you signal to them, only to get them disappointed later. Its a low blow, really.
On the other hand, if a service is expensive, you can be reasonably sure certain types of people will not use such service. Which increases the likelihood of meeting peers of your class. Good for friendship and mating purposes.
Case in point: I have fond memories of going to Playa Montroig as a young kid. One rule there was absolute silence after a certain hour. If violated repeatedly, one could get kicked from the site.
Who is the target market here? The members would all be people who own property near popular vacation sites, which presumably well-off people with disposable income. But those people are not looking to travel and then stay in their cars in a stranger's driveway. They can afford hotels and vacation house rentals.
This seems like the worst of both worlds. People who don't care about the price can afford better accommodations. People trying to travel on a budget won't have properties worth using as a host and can't even participate.
I would guess this maybe targets people wealthy enough to own both property near a popular vacation site and an RV, but who want to travel to other popular vacation sites that don't have nearby RV hookups.
It would arguably make sense as an AirBNB for driveways, as in let the hosts and the travelers be different people with very different demographics, but requiring all users to be both doesn't make any sense.
Dominant case in point is the long discussion here over the $100 price point.
Is this a spill-over of the Open Source ideals many hold? That software, and by extension services should be gratis as well as libre?
People point out alternative (free) sites, and so on, as if the price matters. Or more accurately as if its the only thing that matters.