This is why it's important to match up the expectations of the family with the expectations of an au pair...
Someone who wants a business like arrangement - room, board and pocket money in return for some childcare and cleaning, time to study English and have fun with their friends - is not going to fit well with a family who want their au pair to spend a lot of time with them, playing games, having dinner and going on trips out at the weekend. Those families post on here going 'why is our au pair ignoring us? she's never in!'.
Someone who wants to be part of a family isn't going to do well in a family who want the arrangement to be more business-like. That family would suit an independent au pair. They post on here saying 'my au pair is really clingy, this isn't what we specified - we wanted someone independent and outgoing!'
For those reasons guidelines like 'we prefer to have time as a couple in the evenings' are important because it doesn't raise the expectations.
If my DC went to be an au pair I would expect that family to give them all the legal protection required and uphold whatever end of the bargain they agreed to. If they said that they wanted to welcome the au pair into the bosom of their family and went back on that then I'd be pissed off but the majority of families who have the kind of business like arrangement just don't promise those things in the first place.
Both parties need to be upfront and select their match carefully.
The ones who exploit their au pairs are the ones without a contract, who say they will treat the au pair as a member of their family/give them that expectation and then don't. That's not fair to anyone. Sadly there are plenty of families like that who simply haven't looked into the realities of having an au pair or thought about the obligations. My view is that agencies are to blame a lot of the time as well because they don't seem to adequately prepare families, they don't provide accurate advice on contracts or working hours and they don't check up on the au pair afterwards to check that everything is okay.
There is no longer any obligation at all to 'be a host family' and treat the au pair like your son/daughter. There is, however, an obligation to be an employer. With that in mind it's much easier to have the professional relationship in place first because that is the base of the arrangement - you hired them, you pay them in return for work, you can fire them. Then if you're happy for them to be part of the family you can include them but that's for the family to decide and the prospective au pair to agree to or not.