Shouldn't the tax in this case be of income tax type rather than being capital gain tax type if they are accepting "payment" in cryptocurrency?
Income tax applies when you receive money in the form of income, for example, you do some work and get paid in Bitcoin. You mine Bitcoin, what you receive is considered income. Etc. Basically income is roughly when you receive money in exchange of goods or services.
Capital gains tax apply when your capital (Bitcoin) has a higher value when you dispose of it (sell, use, pay with it, etc) than when you acquired it (bought it, received it as income, etc).
Therefore if you're paying something with Bitcoin, in the eyes of the tax man you're basically selling it for USD, and getting the benefits of it's increased value, which is the capital gain.
Note that you don't get taxed twice (income and capital gain). You first pay income tax if applicable, and then capital gains are considered from that amount of $ up. If the price remained the same, then there's no capital gains tax.
