For those interested, here is why:
"Shinou" has a different pronounciation than "Shin'ou". In Japanese, there's a character for "n" and one for "no". Therefore, in order to avoid confusion, it should be written with a ' sign after the "n". If you write "Shinou", it's actually "Shi-no-u" (シノウ). So, only way to avoid confusion, it has to be written "Shin'ou", which is "Shi-n-o-u" (シンオウ).