I strongly oppose the notion that one can be healthy/fit at every size, and I don't view claims that overweight people should lose weight as intrinsically body-shaming, but I still think this is a good move.

Those who are buying ads on Pinterest for weight loss aren't doing so out of good will and a desire for people to be healthier - they're doing so to generate profit. They have no interest in actually helping a person change their lifestyle so as to lose weight, keep it off, and become healthier, as those people do not make for repeat customers. Also it's harder to sell a "lifestyle change" (i.e. the information one could use to go from being overweight to a healthy weight) than it is to sell ineffective and sometimes dangerous alternatives such as diet pills and plans.