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