You can’t put a price on principles, but there have always been plenty of people willing to try. In the 1980s, when the sporting boycott of apartheid South Africa was holding relatively firm, the casino operators of Sun City would try to lure global sports stars to play one-off exhibition matches that broke the ban. John McEnroe, then at the height of his rebellious powers, turned down one such pay cheque, aged 24, with the memorable observation that “I’ve got better ways of earning a million bucks.” In the context…