USA representation limits
As the USNCO is seen as a process in which potential candidates for the USA IChO team, only United States passport-holding residents may proceed to the training camp from the USNCO. Further, those who are deemed not fit to represent the United States for discipline reasons during the training camp may be barred from selection for the IChO team, although such cases are extremely rare. A maximum of two students from the same educational institution are allowed to participate in the USNCO, even if more students qualify for the USNCO in a local section based on score. This rule prevents a science-oriented school from sending excessive numbers of people to the training camp.
One other special limitation is that a person who has earned either a gold or silver medal in the IChO may not participate again in the study camp. This limitation is put in place purely for the chemistry awareness, as a student who has learned from one year's IChO most likely will do much better in the following year. The rationale for not limiting a person who has been awarded honorable mention or a bronze medal from competing the following year is likely based on the fact that if the USA team was in a condition that there were bronze medallists or lower, then the chemistry awareness may be sacrificed for the performance of the team.
Note that the only official reason that ACS gives for these limitations is "chemistry awareness", and that more detailed explanations are purely speculative, but they seem much more reasonable than any other possible explanations with the information given.