Pearl Harbor, while a surprise attack, was a military target
Pearl was attacked by a national military force, too, not a bunch of organized religious fanatics with boxcutters and a suicide plan.
One could make a case for the Pentagon being a military target. No such case can be made for the WTC.
As a site of massive civilian casualties, and in view of the international nature of the killings, a memorial service was more in keeping.
Regards--
Mary Anne