That is because if you go to Tools -> Options -> Spelling & Grammar -> Settings you will see you can select whether you want Word to check for one or two spaces.In formal writing Microsoft Word says you're wrong if you only have one space between sentences...
Well, you are right that "united" is an adjective and that "states" is a noun. However, "of America" is a preposition phrase.United is an adjective. States is a plural noun. The "of America" is another adjective. Therefore, "U.S." is as valid to refer to the United States of America as "The U.S.", and for us punctuationally apathetic, "The US".