Their reasoning for this is supposedly because their fixed costs are in Euros. Therefore if the price of bitcoin fluctuates, they may not make their money back even if they sell enough tickets, and they don't want to be in a position where they are betting on the price of bitcoin. Ideally, if the bitcoin trading markets were more mature, they could avoid this risk by hedging.

For example, let's say their fixed costs are 21000 EUR, which would require selling 300 tickets to break even. At current prices (8.4 EUR == 1 BTC), 21000 EUR is equal to 2500 BTC. Let's also say that in order to pay back their fixed costs, they must convert any tickets sold in BTC back to EUR at whatever the market price is in the future.

So, assuming they charge 8.4 BTC per ticket, the risk is that the price of BTC goes down when it's time to convert their BTC to EUR. If they sell all 300 tickets, but the price of BTC halves, they may still be in debt 10500 EUR.

If the bitcoin trading markets were more mature, they could hedge against this risk by selling bitcoins short at the time they pay their fixed cost. Then, if the bitcoin price does go down, they will make money on the short sell, and still be able to repay their fixed cost. If the price of BTC goes up - then hopefully they will sell more tickets (since they will be a better deal now), in order to make up for the money they lost on the short sell.

Hopefully next year they'll be able to charge tickets in BTC. Trading markets will hopefully allow more complex trades which facilitate hedging. Also hopefully the price will be more stable, which will reduce the need for hedging.