Not true, here's the sample menu from London 2012 that illustrates what will typically be available at venues inside the olympic park:
Bottled water – £1.60
Fairtrade tea – from £2.00
Fairtrade coffee – from £2.60
500ml bottle of Coca-Cola – £2.30
330ml bottle of Heineken – £4.20
London 2012 red wine 18.7cl – £4.80
http://www.london2012.com/mm/Document/Documents/General/01/2...
Definitely overpriced, but Coke isn't the only option.