Yeah, I did give it some thought before adding to the map. The guideline mentions recurring events and offline use.

 While the event itself is temporary, the event site is permanent. Street layout and permanent infrastructure doesn't change much from year to year. Consequently, there is actual ground-truth to draw from. There are physical things on the ground which remain there year round. They can be seen in satellite imagery, and are tangibly there when you're at the location.

There's no internet there, which indeed means people will be using offline maps which may be some months old. 
People are on the land many months of the year.

While some components like street names change annually, there are a lot of components which remain static. The location is very remote. There's no internet, no water, no shops, no shelter, and a lot of dirt.  Having comprehensive map data available is incredibly useful to people, such as participants, artists, organisers, and even emergency services, who will be visiting the site and need to plan their activities many months in advance.

The annual events held there are indeed temporary in nature (I'll try tag dates or smth), but the land where it's held is permanent. 

 Burning Man's Blackrock City is almost identical in nature. https://www.openstreetmap.org/#map=14/40.7854/-119.2038