Answer:
The biggest argument is that while being U.S. citizens, they do not enjoy the same civil and political rights as citizens living in any of the 50 states.
They cannot vote for president, they do not have voting representatives in Congress, and they cannot access as many federal funds as the 50 states.
In order words, they are almost like second-class citizens and the U.S. Constitution is supposed to forbid this kind of treatment.
