Language peeves are sometimes a bit hard to understand - the strange case of hard and difficult

I recently came across this, from a commenter at the linguistics blog Arrant Pedantry:

Schoolwork (homework) is not “hard”; it is “difficult”.

Comment at Arrant Pedantry

Googling around, I found a questioner at Stack Exchange saying that this had been a pet peeve of his grandfather, which led me to a discussion at Language Log, where a correspondent KF had written:

Spelling is difficult; walls are hard.
Many people fail to use the word hard correctly….

But why should anyone think using hard to mean difficult to be incorrect?