Please don't mistake confidently arguing for patronizing.
"The USA is the biggest power" is a form of context. "The speaker is from [country X]" is a form of context (Churchill, definitely not from the USA, once famously said "You can always count on Americans to do the right thing - after they've tried everything else.") There was adequate context for the original t-shirt to make it clear what "America" was referring to.
Which makes the "not just the US" argument come across as similar to if you argued that "LeBron isn't the only star, Aldebaran is also a star". Virtually everybody knows there are multiple meanings of the word "America"; we don't need to be told that every time we use it to refer to the US. The appropriate standard for word usage is "the same republican principles as American civil and ecclesiastical constitutions" [Noah Webster], that is, popular usage. And in popular usage, "America" can be used to refer to the US, or to the landmass as a whole; arguing against that is like arguing that we should stop calling famous actors "stars" because they're not luminous spheres of plasma. It's incorrect, and also annoying.
"The USA is the biggest power" is a form of context. "The speaker is from [country X]" is a form of context (Churchill, definitely not from the USA, once famously said "You can always count on Americans to do the right thing - after they've tried everything else.") There was adequate context for the original t-shirt to make it clear what "America" was referring to.
Which makes the "not just the US" argument come across as similar to if you argued that "LeBron isn't the only star, Aldebaran is also a star". Virtually everybody knows there are multiple meanings of the word "America"; we don't need to be told that every time we use it to refer to the US. The appropriate standard for word usage is "the same republican principles as American civil and ecclesiastical constitutions" [Noah Webster], that is, popular usage. And in popular usage, "America" can be used to refer to the US, or to the landmass as a whole; arguing against that is like arguing that we should stop calling famous actors "stars" because they're not luminous spheres of plasma. It's incorrect, and also annoying.