They're not there as spectators, they're there for security. They need to be facing the potential threats. (I found that out by googling "military turn backs to motorcade fact check").not quite in the threads realm but post WH riot the claim is Military turns backs to Joe Biden's inauguration motorcade.
A video showing the inauguration motorcade for President Donald J. Trump in January 2017 here appears similar, with some facing away. A video of President Barack Obama’s inauguration motorcade here also shows various guards facing away from the road.
Nahaku McFadden, media operations chief for the National Guard Bureau of Public Affairs, confirmed to Reuters via email that the video shows routine protocol for this type of duty.
“These National Guardsmen were on duty with a mission to protect the president against potential threats,” she said. “Some are facing out to ensure the safety of all.”