Read the previous verses. He's put to death whether she is willing or not willing. Her willingness does not matter. What matters is that he's violated ANOTHER MAN'S PROPERTY. If she's single, he can marry her. Rape or no.
It doesn’t say it’s because she is another man’s property. You are imposing that on the text.
A woman is generally under her fathers headship or her husband’s headship. Sometimes they are not, like Naomi and Ruth, due to deaths or perhaps abandonment. Regardless being under the headship of your dad or husband does not mean they OWN you.