Gotta make note that when those graphs are broken down into causes of death, the low BMI folks are almost all dying from respiratory diseases, and the #1 is lung cancer.
Not true, you find the same results when you control for smoking. The seven countries study did exactly that and found, among non-smokers, that being overweight (not obese) had no effect on mortality compared to "normal" weight people. Most of what you read on the risks of excess weight are either BS or grossly exaggerated.
There's also a double standard where the health police say you have to control for cancer in the thin group. But they don't want to allow for that in the analysis of the overweight and obese group because they claim excess weight causes cancer...