Because it allows the agency to escape from its bad design problems by pushing the (huge) cost onto its clients -- and those clients are other parts of the US Govt or funded by the US Govt.
You're asserting the design is flawed when that's in dispute.
It's useful for those agencies' budgets to reflect a portion of the cost of performing that research.
The USG needs insight into what taxpayer dollars are being spent on. Lawmakers have to explain to constituents why that money is being spent.
NASA is the first tier of information, collecting the data. Its budget ought to reflect that cost.
The consuming agencies are the second tier, processing that information. Their budgets reflect the cost of gathering their information and of processing it.
NASA doesn't know which information will be useful, so it's not helpful for them to pay the cost of egress. We want them to collect as much as possible.
It's much like a music store, 90% of their sales come from the top 10, but there's a lot of value in hosting obscure stuff.
If they have to pay to store it all rather than pay for egress, they'd have to justify the cost storing data that they can only say "it might be useful some time."
Having the agencies that are working with the data pay for the egress, they can justify the cost by showing the specific work they do.