Hermione's parents I'm sure are well aware that there is no way for them to stop Hermione from doing what she pleases

I completely disagree. Not many parents would be able to accept this, particularly of a teenager; hell, most adults I know won't accept it of their grown up kids! But also again, affection and loyalty issues. They might know they're not be able to physically stop her but I can't imagine they wouldn't believe they could talk her out of it one way or another.