The Post Office has said it will no longer stop US citizens from applying for a cash Isa

The Telegraph in UK writes ;

"Among those who would not have been able to take out a Cash Isa with the Post Office is Boris Johnson, the Mayor of London, who was born in New York and holds a US passport.
Under Fatca banks must report the names and financial details of US customers to America’s Internal Revenue Service (IRS).This is costly, which is why some organisations, including National Savings & Investments, have decided not to comply and have instead closed the accounts of US nationals.
Other innocent Britons are also caught up if they own trusts, for instance. It appears the Post Office was adopting this approach – even though Isas should not fall under Fatca’s reach.
Following our questions the Post Office has changed its stance."

The comments are mixed. Remember, this is from the UK.
One comment was very interesting.... Seems that If W was tried as a war criminal in the Hague then the USA would have invaded Netherlands. I didn't know that.