Nice job ignoring the second part of my sentence that put it in context.
USD isn't irrelevant alone, however when being part of a programme based around another currency, it is.
If I open a savings account and put £100 in, I wouldn't measure the success of those savings in $. Due to exchange rates and the like, it would not give me an accurate representation of how well my savings account is performing (especially so if £ was up against the $).

The exact same thing applies with programmes like NastyFans. Seats are bought using Bitcoin, so then representing the programme's performance using USD is both inaccurate and dishonest.