The US refuses to explain to a British couple why they were refused entry to the US for their honeymoon, despite having approved them in advance.

I don't think any country is obliged to accept all visitors that wish to enter. There are various legitimate reasons to keep someone out. But the US demonstrates unreliability as well as incompetence by giving people visas and then blocking them by surprise at the airport.

Why take a foolish risk? Don't visit the US. Go somewhere else which won't screw you over.