What people forget is that not all choice is equally easy.
The choice to "diet and exercise" is easier for a rich guy with a personal chef and trainer.
It's much more difficult for a poorer person working 2 stressful jobs, having to go back in a home late in the day exhausted and deal with whatever family/personal issues lurking around.
And while there's a spectrum between one and the other extreme, more lower middle class, working class people are closer to the second case. (And there are circumstances in the first case, like stress, addiction, psychological history, that can make it hard to diet/exercise, even if you could easily afford a personal chef/trainer).
So, yeah, it's a choice.
But what (budgetary, stress-wise, family, expert, time, etc) support a person has in making that choice and sticking to it is not the same in all case, and can vary widely pro and against.