Convention is the antithesis of art.
This trend had nothing to do with the fate of the person being depicted on horseback. As a piece of historiography, that piece has no merit. Absolutely no evidence is offered that there ever was a code or that it was reinvented. But the myth is so persistent that one can't help but suspect that some fourth-rate funerary artists may have gotten the word. There are also numerous reports that this convention originated or was followed at the Gettysburg battlefield.