Unless state laws specifically adds further restrictions, US Federal law permits Canadians who enter the US legally without a visa as non resident aliens to legally purchase firearms, according to the BATFE.
People may have been able to purchase a firearm but they were breaking the law. You can only purchase if have been a resident of the state for 90 days. 90 days as a visitor does not count.
https://www.gpo.gov/fdsys/granule/CFR-1997-title27-vol1/CFR-1997-title27-vol1-sec178-11
Page 907: "Temporary stay in a state does not make the state of temporary stay the state of residence."
And if you're not there temporarily then you broke the law when you entered the USA by not declaring your intentions to stay.


















































