Why are people living in the same countries being required to pay in USD when they are both resident

[Mark625]

Why are people living in the same countries being required to pay in USD when they are both resident outside the US?

E.g...host and guest in UK...the guest is booking with a UK debit card but right at last minute, the sterling is being converted to USD instead of going through as sterling. 

I see this is happening with other people in various countries.

Makes no sense ...is this a corporate decision by air bnb?? Is it a way of making extra cash fo rthe company?

Mark