Thrust,
Actually, what you are suggesting is technically called "Cabotage".
This is the big advantage that USA has over every other competitor in the world. The size of the domestic market is so big that they have to protect it otherwise their market would be decimated by high quality service by asian and european carriers.
By all means you can have 5th freedom between Vancouver and New York but not cabotage between LA and NY.
Realities of life do have to come into play.