The people had over an hour and a half. Things like this travel over the internet in the blink of an eye.
I understand very well what you are saying, but I might ask you why tornado warnings are posted on the internet in the United States? Surely that is not an adequete way to convery a warning.
I used to live in Tornado Alley, and I still live in a tornado-prone area. You get your warnings from TV or radio after you notice the fact that the weather outside looks crazy. Even then, you don't really duck for cover until you hear the sound of a train in the distance.
All of the nations involved with these tsunamis are second or third world countries. A 90 minute warning would never have been conveyed to the people on the coast. A three and one-half day warning might have been helpful.
Understanding the history of the place, where nobody has any written record of anything like this happening before, I am not surprised people in the region didn't think about it.
Civilized countries or not, there is no way that with 2 hours warning they can get the message and implement a plan. If anything, a hastily issued warning would just cause mass panic and likely cause more deaths.