'Hiring for free' - oxymoron or HR inspired nightmare?

Judging by the BBC report, Sainsbury's in Camden look to have got the kicking they so richly deserved by advertising for a local artist to decorate the store's staff canteen for free. But when did expecting things to be done for nothing become an accepted part of the employer-employee relationship or commercial life in general? Is this something that has crept in from the egregious practice of unpaid work experience/internships? Or has the employer-employee relationship now become so slewed in favour of the former that the latter is expected to receive no consideration in return for expending their skill and labour? Either way, the practice is contrary to the rules governing formation of a contract - where there is a presumption in favour of creating legal relations where a person (be they employee or contractor) is taken to have entered into a binding contract if the circumstances are those where payment is usual. In this case, an artist would expect payment for carrying out a commission (or a painter and decorator would expect to be paid if they painted the walls or hung some wallpaper). I suppose you could try to see if the opposite applied: see how Sainsbury's would react if several hundred Camden folk entered the store, loaded their trolleys and then left without paying?