The rationale is that it's part of the shift to large-scale testing of passengers, but you have to wonder.
Maybe a move to embarrass Biden, who will no doubt reimpose the ban? That would also piss off a number of people.
The airlines have been pushing it for some time; maybe he's angling for free lifetime trips, now that he won't have access to Air Force 1?
A more cynical assessment would be that, as we've just crossed the 400K mark for Covid deaths here, the day before he's finally gone, he wants to ramp up the numbers for Biden's first months, making his own "accomplishment" in that area not look so bad.
But nobody could be that callous toward human lives. Could they?