Sorry, but I have to inform you that it's called the Open Championship, and it is THE Open because it was first by a long shot. First played in 1860 before there was even a golf course in the US, it predates the US Open by 35 years. It still is the preeminent major championship in the eyes of many. Understand also that Britain is steeped in history and tradition, and it only stands to reason that those qualities would be an integral part of the game which began there. I can be a homer and root for Americans while still not feeling diminished by recognizing THE Open for what it is - one of the truly great traditions of our game.
It's no more incorrect than for MLB to call their championship the World Series and declare the winner the world champion. It is like that because that tradition began in the US. Well the tradition of an open championship in golf began in Scotland, so they retain the right to the name.
I'm no xenophobe thumping my Red, White, and Blue "These Colors Don't Run" shirt on my chest, trust me, and I greatly respect the history of golf as a UK sport, but this is the 21st century and there are 4 majors that most people believe have equal weight in importance. If England created a baseball league that attracted the best talent in the world from the Caribbean, Latin America, and Japan and they called their championship game the British World Series, I would probably lobby to make the US call their baseball championship the US World Series, or something like that. Currently no baseball league in the world compares to the US one, so keep it the World Series. If baseball players traveled around the world and played in 4 championships of equal importance, I'd expect none of them to be the exclusive "World" Series, even the US one.
This is really no big deal. Maybe my Irish blood just got to me! Hearing "The Open" just makes me think of that Robin Williams comedy bit about the history of golf where he says "...and then you see these old WASPy f***s declaring "Dear God, how did the help get to play?" or something like that. Hope to see you at St. Andrew's some time!