Because torturing peasants and burning villages is for most people low-level medieval fantasy (though not so much these days, sadly), while rape is something that they say has happened or will happen to 1 in 3 of the women in the room you're sitting in right now?
In short, the "immediacy factor".
That's my feeling actually about it. Lets be honest, Battlemaster is not played strictly as a medieval simulator. It's played by people from the modern era and that will come through at times. Murder and death are of course bad things, but we have a back drop of seriousness we can put it against. Rape... a lot of people still trivialise it to a great degree. Plus there is a considered to be different levels of murder too (killing combatants in war as opposed to killing in passion or killing civilians), with enough differentiation between them in all eras. I can say that my particular knight does not like killing civilians or collateral damage, but being a warrior is his profession and war is an unhappy business. The argument "is rape worse than death?" comes up a lot in the real world.
Final note: a lot of people insinuate that things were simpler, more absolute. They weren't. People have never been simple.
