I suspect that you are from outside the US. We have traveled for years with my daughter and we have never paid a third person supplement.
Obviously, the situation is different in Europe. Doubles tend to be so small that the hotel will not allow a third person -- period. Also, there is no requirement to show passports in the US.
I agree -- even for Las Vegas you are getting a very good rate. Don't go to the White Tiger show, its a rip off.