Why is the seller requesting a USDA loan when I'm approved for FHA?

I found a house I am interested in buying, and submitted an offer. They countered my offer and said they wanted me to go through their lender to get a USDA loan, even though I am qualified for an FHA loan. I countered back that that was unacceptable, but they still want the USDA loan. The sellers are professional "flippers," and I suspect something fishy is going on here, though I don't know what. Why are they concerned with my loan when I already have financing set up? What do they have to gain by being so stubborn about this issue? Any help is appreciated!