Seems like sheer stupidity but people who are disadvantaged in life take risks - I feel for the pain being suffered now by many family members resulting from this tragic accident.
I will be the devil's advocate here; "tragic accident", War and the effects of war and all the deadly sh*t that is a part of it, is only for the purpose of killing people. It has been about 73 years sense WW II and that war is still killing innocent people today. The Vietnam war killed an estimated 4 million Vietnamese and they are still dying today from explosive material, left over from that war. As for "Seems like sheer stupidity". I know you said "Seems like" but I must disagree with your choice of the word "stupidity". No doubt they were ignorant as to what they found and the danger of that mortar round. In my eyes, just because they did not recognize the danger they were in, doesn't make me look at then as "STUPID". I can not imagine trying to live the way these people do, in the country here.
I agree with your final sentence (I think I made that obvious in my posting) and I used "Seems like" to indicate that is how it may SEEM to others, even though it is NOT the case. If I wished to infer they were stupid I would have said "I know it was sheer stupidity". If you wish to analyse the words used on this thread to judge in moral terms what posters meant then I suggest there are other postings which may bear fruit if you bother to analyse them.
There is a difference between "stupity" and "unawareness" or "ignorance". When someone does not know what a bomb is, they will handle it different than someone with knowledge, what a bomb is. It can be that the schools are terribly failing in this case. Even in Switzerland the teacher teaches what a bomb is and how to handle the case.
People are suggesting that there are still some living in the conflicted area of Zamboanga who do not know what a bomb is. Considering the history of violence in that province it is hard to believe that anyone who lives there could reach adulthood without knowing what a bomb is.