Dissimilar Redundancy. Say the Falcon 9 suffers an in-flight failure. The last time that happened, there was a stand-down for quite some time, and I suspect that there would be a similar amount of time before people could be flown on it, even if I'd think Starlinks could resume right away.
Starliner/Atlas wouldn't be affected, and so ensure the US and our allies have access to the station without getting Russia involved.