As a Canadian I share your amazement at how Americans seem to think Canada is the 51st state. Americans are constantly being stopped and sometimes arrested at the border for trying to bring guns in the country. Generally they aren't deliberately flouting the law, they seem unaware that they are entering a different country that doesn't have a second amendment and has different laws respecting guns. A particular peeve of mine is the Americans who, at each presidential election, go on about how they are moving to Canada if their candidate doesn't make it as if we're just another neighborhood rather than an independent nation with its own immigration laws.I'm appalled by how many Americans seem blissfully unaware that Canada is actually a separate country, but merely baffled by how even a small number of Canadians could apparently be so stupid.
Not a problem we Canadians face in respect to visiting the United States. Your Homeland Security agents generally make it pretty clear we're being allowed in on sufferage.