I've said this before on here, but I think it actually makes the most sense. If the Nats want Harper to play CF or RF, he needs to learn those positions. And Eury Perez and Destin Hood, the CF and RF at Potomac, aren't ready for promotions. All three need to play everyday.
So Harper either was going to skip high-A or one of the three was going to have to play out of position in LF, which wouldn't help any of them.