[vsef]

No, it's not a US thing. Cheddar melts fine, but the harder and more mature the cheese, the more the solids and oils will tend to separate and not be as super smoothly emulsified if you melt it onto noodles on its own. The point of a roux (traditional, French cheese sauces), sodium citrate (modern), or using processed cheese (American boxes, what Americans are thinking of texture-wise when they think of mac & cheese) is all to get the cheese to be very creamily emulsified. Processed cheese is just curd dregs + emulsifiers. Just melting cheese is fine but its not the same effect.