Cuz... emperor, the son of God only answerable to God.
a) It's not always non consensual
b) Because that's just, how it was back then. Not saying, it was or is right, but most of these historical manhwa (or manhuas for that matter although they tend to leave that part out or only imply it because, well, China) depict relationships between people of vastly different statuses and if you had the unfortunate luck of being a peasant, anyone above you in status (doubly so if the other person is a highly ranked official or prince or emperor) could rob you, rape you, torture you, kill you, do anything they want to do to you without repercussions, so that was just reality back then (and let's be honest, 90% of the time that's still how is to this day, if you have enough power and/or money.)
Yeeeah good point
Why is sex in historical manhwa never consensual