IANAL, but I'm pretty sure it is legal. As a general rule, people are free to accept or decline whatever payment methods they want, with the exception that legal tender must be accepted as a settlement of pre-existing debt (though other methods of payment may be accepted if the creditor agrees). Compare how eBay forces sellers to accept PayPal, which is also perfectly legal, apparently.
But the Olympics is not just anyone, they are state sponsored entitiy.
IMO, this is an illegal state aid under EU single market rules.