BMI isn’t fatness. BMI is just weight / (height^2), which means that BMI grows linearly in proportion to a human’s scale. (Weight grows with the cube of dimensions but height^2 only grows with, well, the square.) BMI also fails to distinguish between muscle, bone mass, and fat. If, say, human heights had been increasing over time (while maintaining other bodily proportions), you’d expect BMI to increase over time.
That’s not to say humans aren’t getting fatter, but if you wanna hypothesize on the causes of trends in human fatness, you need data on human fatness. Not BMI.
Legalizing X doesn’t just mean you can do X if you want to. Legalizing X helps normalize X, and gives other people license to expect you to do X. When child labor was legal, society could expect the poor to rent their children to mines and factories. Were the poor forced to rent their children at gunpoint? Well, no. They were coerced by economic circumstance, as usual.
If you legalize a way to mitigate desperate poverty, then the desperately poor can be expected to do that. And those who refuse will be seen as unsympathetic and unworthy of assistance. After all, how can you say you’re really poor? You have an idle ten-year-old and a perfectly good spare kidney!