Github from a political standpoint cant drop one govt agency because they stand to lose all their other govt contracts. And any other buyer who now has to now consider they will be dropped if they come down on the wrong side of Github employees' moral compass. Github as a business, especially one now owned by Microsoft cannot do that.
I think Github has tried to do as much as it can to support policy reform without irreparably damaging their business. If that is still not enough employees need to leave at this point. There are many others who would love to work at Github and can reconcile with the situation that Github is in and accept that they are trying to change policy rather than drop the agency and hurt their business.