Surely this is an offence against Civil Liberties? If I'd been found
guilty then maybe I could have understood it but in these
circumstance this seems totally unfair, especially as I've previously
travelled to the US many times without problems.

That is the US requirement and always has been. AFAIK you have always had to
declare this so you may have problems explaining to the US Imigration
service why you did not do so before. You need to obtain a copy of your file
on the Police National Computer. You can get an application form at any
police station.