The DOE is doing the work because desalination takes huge amounts of energy, and they're looking at ways to reduce this.
Everything that happens takes energy. Is the DoE going to try and be experts in all of it? They'll have the broadest mandate in history. The consumers of the energy are in a better place to direct the research.
But hey, the US is adding a trillion dollars in debt every 100 days, so the DoE may as well do this thing. There isn't enough money in the pot here to have a serious argument over it.
If you look at, y'know, energy use in the US [1] then desalination doesn't even rate a mention as an energy sink. The biggest source of waste by far is the 66% losses during energy generation and in electricity and transport. Realistically they should be focusing on the supply of energy.
And to be fair they probably are. I'm repeating myself, but this looks like the bureaucratic equivalent of someone with a bee in the bonnet and nobody could be bothered to tell them no.