There’s clearly a pragmatic incentive to allow reused rockets. The Space Force expects to save $52.7 million for the GPS III missions alone. It might also be difficult to insist on brand new rockets. SpaceX is shifting its focus to Starship, and might not be eager to make more Falcon 9 rockets than necessary.
This also reflects added trust in SpaceX. Although the company has clearly played a crucial role in US government launches through projects like Crew Dragon, the contract represents another level of confidence.