Why is the U.K. not included? This is so weird; it's such a mess in the UK too.
Even if they don't restrict it to British passport holders travelling to the USA, it's still arguably easier to screen for the virus through one departure point than, say, 30.
The travel restriction applies to foreign nationals who have been in 26 European countries with open borders agreements, in the last 14 days.
UK is not included because it has never been a part of Schengen ("26 European countries"), which is targeted for refusal to impose travel restrictions.
I’m honestly not sure there’s anything more to it than that.