Probably based on principle. You shouldn't be swearing to your parents and he said it was disgusting for his daughter to swear - not for you to swear in general. He just doesn't want you swearing whilst having a conversation with him. It's a respect thing, or that's how it is in my family.
I can't swear whilst having a conversation with my mum, but if my cat scratches me and I say that it ******* hurt, she doesn't care. As long as I'm not directing the swear words to her or using them in a conversation with her, she isn't bothered.