Why is not payment made as waves? This is so weird. If you do pay with Waves, it will benefit both you and the users.
Maybe because they want to post this thread to the Bitcoin's Services section or else they will be posting this in Altcoin's Marketplace if they want to pay with Waves. And maybe because most of the buyers of altcoins was came from bitcoin isn't it?
I think the most important reason for not doing that is because people will, for sure, sell their Waves to Bitcoin which will cause a lot of sell orders, below the lowest one available at that point, to get quick BTC. It will hurt the project greatly.
They could give the option to receive the equivalent of the BTC payment in Waves instead, but it is their choice. You can use the payment to buy waves if you want.