That would have been atrocious especially now, but back then that law was made to ensure that the father was compensated for the fact that the rapist tainted his daughter/currency and that the "soiled product of trade" was still taken care of as far as being fed, housed and clothed.
Plus, she had the one in a lifetime opportunity to throw any fit she wanted to her "husband/rapist" (something women couldn't do in an average marriage/trade in fear of severe punishment or even the death penalty) and he couldn't divorce her no matter what she did or said.
Still sucks to think women were treated as objects, even lower than cattle back then.