What is the original legal basis for the decision to prevent the people of the United States territories from full citizenship?

they were not able to understand the principles held by USA

Explanation:

During the time when USA was considering the option to opened up Immigration path from another country, there are one main advantage and disadvantage that divided government officials' opinion regarding immigration

- On one side, immigration created an influx of workers that will positively contribute to USA's economy. It will allow the nation to utilize lands and resources available in their territory and generate their wealth through taxations.

- On the other side, some government officials worried about the cultural difference between the immigrants and the locals.  Cultural differences tend to influence the personal principles which determine what each group considered as 'right' or 'wrong'. They're worried that the difference will created conflict between the locals and the immigrants and turn the country into chaos.

the answer would be in this answer. someone already asked the question.