Yes, military tribunals for suspected terrorists were justified because those committing these offenses caused great harm to U.S. citizens, and effectively declared war on the United States. The United States was justified in treating these people as the combatants that they are, with the facilities and systems designed to deal with enemy combatants. A tribunal is not inadequate or unfair simply because it is a military tribunal. Rather, the tribunals dealt with these individuals as the enemy combatants that they were.
No, military tribunals for suspected terrorists in the United States are not justified as each person should be given the right to due process, something that a military tribunal negates. As such, it is a violation of human rights, and cannot be considered to be justified in any sense at all.