There is no legal requirement to be a US citizen to purchase ammunition.
The recent ruling made this abundantly clear.
As for myself coming from Canada which does not require a visa I am perfectly legal in purchasing ammunition.
https://www.federalregister.gov/articles/2012/06/07/2012-13762/firearms-disabilities-for-certain-nonimmigrant-aliens-2001r-332p
In 2002, the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) published an interim final rule implementing the provision of the Omnibus Consolidated and Emergency Supplemental Appropriations Act, 1999, relating to firearms disabilities for certain nonimmigrant aliens. That regulation implemented the law by prohibiting, with certain exceptions, the sale or disposition of firearms or ammunition to, and the possession, shipment, transportation, or receipt of firearms or ammunition by, nonimmigrant aliens.
The Department of Justice has now determined that the relevant statutory prohibitions on transfer and possession of firearms and ammunition apply only to nonimmigrant aliens who were admitted to the United States under a nonimmigrant visa, and that
the prohibitions do not apply to nonimmigrant aliens who lawfully entered the United States without a visa. The Department is therefore issuing this rule to make conforming changes to the regulations, so that the regulations are consistent with the Department's current legal interpretation.
This final rule addresses only the nonimmigrant alien visa issue. The remaining issues raised by the 2002 interim final rule, and the public comments submitted with respect to those issues, will be addressed in a separate forthcoming rule.
"Upon review of the comments and in light of the OLC opinion, the Department is issuing a final rule that applies to the firearms disabilities in section 922(d)(5)(B) and 922(g)(5)(B) only to aliens admitted to the United States under a nonimmigrant visa, as that term is defined in section 101(a)(26) of the INA (8 U.S.C. 1101(a)(26)). Nonimmigrant aliens lawfully admitted to the United States without a visa, pursuant either to the Visa Waiver Program or other exemptions from visa requirements, will not be prohibited from shipping, transporting, receiving, or possessing firearms or ammunition, and the regulations will also no longer proscribe the sale or other disposition of firearms or ammunition to such nonimmigrant aliens"