Yeah, duty of care is an interesting concept.
Party A says that Party B has to ensure the bodily integrity of Party C, regardless of the wishes of Party C.
I can't build a huge, exciting, but potentially dangerous amusement park, and allow admission only to adults, and only with the stipulation that you ride at your own risk.
You have rights that you can't waive. In that regard, Party A is the one treating you as a pet, interjecting in the relationship between Parties B and C.