I learned some basic "Espanol" in a Texas classroom, and we didn't necessarily learn "European" Spanish but more "New World" Spanish, since that's what we'd encounter around here. Some of the little rules (like the capitalization you noted) would be completely missed by (and probably weren't taught to) elementary and junior high kids not fully immersed in the language - many have a hard enough time with the rules of the language they grew up with!

Since I was unaware of that rule until now, if I were to see it in lower case, I'd figure the writer was either lazy (most people these days) or heavily influenced by e.e. cummings.
And, on the signs I see, they are either all caps, or use uppercase for "Español", so still incorrect based on the rule.