:::::I am contending that every grammar guide and dictionary of American English I have checked shows "American" is an accepted demonyn while "United States" is only used as a noun. This has nothing to do with your ability to extrapolate a new word usage from the first part of a proper name.  Are you contending that your personal knowledge and expertise in American English is superior to publishing houses such as Houghton Mifflin, Merriam–Webster, Random House, and Simon & Schuster? ''''