"There is a big difference between the loss of 3000 people due to a deliberate attack on your country and the loss of 300,000 people due to heart attacks."
Since you obviously don't mean the heart attacks are worse, and we obviously don't agree, please support this rather than just asserting it. If it simplifies matters, first support that losing one person to deliberate attack is significantly worse than losing one person to illness. Then we can look at whether "significant" is significant enough.
One thing we can quickly agree on is that deterrence (or lack thereof) is more relevant in the case of an attack. But deterrence is only one kind of prevention. I don't see that the actual badness is greater in the case of the attack-caused death.
One way to measure the impact of a death from illness vs. a deliberate attack is the impact on society. Since the goal of terrorism is to destabilize a society and instill fear, the ramifications reach far beyond the individual who died. Take a look at what happened to the stock market after 9/11. The resulting fear was widespread and very disruptive. The ramifications of doing nothing when someone dies of a heart attack are minor, doing nothing when an external force kills a citizen can be far reaching.
The second reason is justice. A person dying from a heart attack is a "natural" death, nobody (except possibly the victim themselves) contributed to the death.
Using the logic of the grandparent, the world should have said "Let's ignore the Rwandan genocide because more babies die of diarrheal diseases."
How would you have reacted to 9-11? Just shrugged your shoulders and said "it's only 3000 people?" Maybe the same for Pearl Harbor? If so, you'd likely be speaking German or Japanese now.
Since you obviously don't mean the heart attacks are worse, and we obviously don't agree, please support this rather than just asserting it. If it simplifies matters, first support that losing one person to deliberate attack is significantly worse than losing one person to illness. Then we can look at whether "significant" is significant enough.
One thing we can quickly agree on is that deterrence (or lack thereof) is more relevant in the case of an attack. But deterrence is only one kind of prevention. I don't see that the actual badness is greater in the case of the attack-caused death.