I remember when Ron Paul was booed for trying to explaing this during a Republican debate 12 years ago. Maybe some people thought he was in some way justifying the attack on 9/11, but most people simply didn't want to hear anything but the simple, jingoistic reason for why al Qaeda attacked the United States.
It's interesting that this letter has come back up as the conflict between Israel and Palestine has erupted. But too many will now dismiss it as propaganda and call anyone who brings it up a Hamas sympathizer.