I think you would be surprised at how few people the BBC actually employ. Those they do employ as full time, permenant employees are unlikely to be in SCD. They are lawyers, HR personel, facilities managers and so forth. Even the likes of camera operators, production managers etc are usually on fixed contracts rather than faling into the definition of employee as we would normally understand it.
Thise booked for SCD are usually self employed and booked for the duration ( pros and celebs). So in a sense everyone booked for the show is an employee for the duration and everyone booked is not an employee by normal definitions.
If you mean should they not book anyone who appears elsewhere on the BBC., I think that would be rather impractical and possibly a restraint of trade for the self employed service providers - aka celebrities.
Besides its not a real competition. They get a fee and there isn't a prize (just a bit of glory). The BBC aren't FIFA or IOC, they are only responsible for making the best TV they can. Given the ratings, I think for SCD they succeed.
OP if you want to see real dance competition with real arbitration then I suggest you enquire in the Latin and Ballroom thread. I am sure those posters will guide you to a resource for real competitve dance competitions.