Donald Draper wrote:
I am not sure when the reverse \S/ was introduced but when Bizarro first appeared he had exactly the same costume as Superman. It was that way in all his 60s, Silver Age stories.
I always thought the best explained is that Bizarro later wears the shirt inside out.
It was a 1960's comic story whose name I don't recall, but can be found in TPB called "Tales of the Bizzaro World". The population kicked Bizzaro no. 1 off-planet when it was discovered that he allowed "perfection" to exist there with the correct "S" insignia, and, after a series of blunders with Bizzaro Luthor, No. 1 returns to his planet and gives everybody new shirts with the S backwards.
It was Bizzaro Luthor's idea.