Well Turkish nationals can no longer au pair in the UK anyway but that doesn't have anything to do with this decision as the system changed before this judgement. As strix pointed out most au pairs are EU nationals or on the youth mobility scheme and therefore have exactly the same rights as far as working is concerned as anyone else. The only official 'au pairs' left are Romanians and Bulgarians who come over on a specific visa. Everyone else is entitled to be treated as a citizen of the UK would be WRT working rights.
There are 2 different points of law here. The first is the employment side which covers stuff like notice, holiday pay, sick pay (if you're above a certain threshold), protection from discrimination. The second, which is what most people confuse with the first, is the right to NMW. Because an au pair lives with the family, is fed and housed by them and joins in the daily activities they are exempt as is any other worker who has accommodation provided.
I don't see a conflict here. It's an obligation to treat au pairs fairly and allows them to access the protection afforded to everyone else by the legal system.
Until the Government reintroduce a specific agreement you can't make the distinction between a foreign national working as an au pair and one working in McDonalds because the same distinction doesn't apply to British citizens.