The United States is physically inseparable from Canada and Mexico. So you're rationale is completely bullshit.Sorry if thats harsh, I fucking hate when people ask that question. Its like why we named it North America instead of just calling it The Land Around The United States, or why we don't just call the entire Caribbean "Hispaniola".
Okay, let me straighten this out. If the United States were floating around in the ocean, Mexico didn't exist, and Canada were just a massive chain of islands above it - would people go back in the late 1900's and suddenly decide "The United States isn't a Continent - every island within a thousand miles must be counted as well; So it will henceforth be known as Islandia?" It's like getting pissed at someone for thinking Pluto is a friggin planet in 2007.
Just to clarify why I would think it's a continent "in this day and age" - Every Geography class in my life had Australia listed as a continent. I'm not saying they were right or anything, just that it isn't that big a stretch for someone to still think that. Seriously, of all the things to be pissed at - why this?