I simply couldn't understand why there is a shojo manga aimed at... girls... featuring a guy hitting a girl... and the girl being submissive about it...

This is definitely not a manga for younger people. You must be at least 15 or above I say, or have a pretty mature mind to read this. Partially because of the submissive girls being hit thing, and the other half the amount of mature content it had. To be honest I was actually quite surprised that it had bits of mature content in it. While it's not frequent, it stands out sometimes, and this was one of the reasons why I was hesitant in recommending this manga to a friend.