You're unrated when you join (no rating).
U1600 is free to join by 1300 rated players... it's just most of them join the U1400.
You should be free to join any section, although the TD may try to put you in a section with players your level by asking how good you are, whether you have a FIDE rating, etc.
If you're not currently rated by the USCF and you enter a USCF tournament, would you be able to sign up for any rating class (ex: U1500 or U1000) or would you be given an initial rating when you register with the USCF?