As Americans, we celebrate the 4th of July because it is the day in 1776 we adopted the Declaration of Independence which was finally signed by all members on 2 August 1776. The colonists felt that they wanted to become independent because George III, the King in Great Britain was raising their taxes and increasing the rules that they had to follow.
The reason why I think it is obvious is because as Americans we would not celebrate the Independence of Guam or other territories that we have.
But don't worry there will be plenty of celebrating here on the bases, but they have made it only a base event because of this planned terrorist attack. http://www.eadt.co.uk/news/man-charged-with-planning-terrorist-attack-on-us-military-bases-in-the-uk-1-4161717
All the Best!
American to Britain