Because the sponsors pay a good part of the fee for the Olympics. You can bet that he wasn't wearing that watch because he bought it the day before but was being paid to wear it and if that stuff happens then the official sponsors get upset and pull out of future events.
It is called ambush marketing and Nike are one of the best at it.
If you can get access and would like more analysis, I would recommend Gruen Sweat on ABC (Australian)
