Using "Americans" in reference to U.S. citizens is a bit like using "Indians" in reference to Native Americans or "cripples" in reference to the physically disabled. I know there are people who find it silly and irritating that they should be asked to use more "politically correct" language, but whenever possible I think it's best to be sensitive about these things. I don't see a downside to saying "United States citizens." It's precise, unambiguous and offends no one. That would be my preferred usage.
What about immigrants who haven't gotten citizenship yet? With any name, someone will be offended. Besides, do you think the Canadians are really offended by it anyway?
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