i think they don't accept them yet because it is against their TOS
They could just change the ToS themselves. Why are they restricting themselves? I speculate it is due to financial reasons and refund issues. Might be a hassle?
I think because of bitcoin is still new ( not all of countries in the world give legality using bitcoin) and the bitcoin's price is high fluctuating, they will not take high risk by using bitcoin for payment method.