Sometimes badges have stuff on the back like card codes, punch centering marks and magstripes that make printing on the back a bad idea.
In Boeing case, its because you are not allowed in many places without a badge somewhere above the waist that is visible. Higher security employers elicit higher security practices from their employees.
Most security cards I've been issued with are laminar - there's a thinner printed front part glued to a thicker RFID back part. So making the card double-sided just means printing two fronts and sticking them on both sides.
I don't see what you lose by printing both sides.