Its pretty straigforward actually. When 9/11 happened some tapes of Bin Laden were released that shows him taking credit for the attack and explaining their reasons. It doesn't mean he was justified, it doesn't make it right by any means.
But we would be stupid not to pay attention to the motives of our enemy. Very clearly, he claimed that 9/11 was retaliation for the santions the US placed on Iraq which contributed to the death of hundreds of thousands of children and for putting military bases on their holy land.