They are a whole lot less expensive than tracking customer usage and billing for it, and a whole lot more useful to the public than having the data nominally publicly accessible but only "on display in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying Beware of the Leopard." [0]
[0] Douglas Adams, Hitchhiker's Guide to the Galaxy
There's likely some truth that CSV would work well here, and would likely be cheaper to operate. I wouldn't be surprised if a lot of clients are or could be doing full transfers of the data and doing their own queries.
I'd be pretty happy with sqlite dumps too.
I don't really have an issue with the REST, though. I wouldn't be surprised if this was just a standard and cheap to set up Django+REST libraries stack. Yeah, the compute costs are higher than transferring static files, but I'd be shocked if this was taking enough QPS for the difference in cost to make a meaningful difference.
I get wanting the government to be responsible, but this veers a bit too far into Brutalist architecture as an organizational principle.
[0] Douglas Adams, Hitchhiker's Guide to the Galaxy