Having a BMI* greater than 25 is a risk factor for heart disease, diabetes, stroke, and cancer.
This move is guided by feelings, not facts.
*Yes, everyone knows by now that BMI doesn't apply to people who are very short, very tall, or bodybuilders. There are other measures like body fat percentage or waist-hip ratio that can also be used.
1) Shaming is a bad idea, even in that case. We see this in all manner of public health policies - shame isn't particularly effective as a motivating tool. It also leads to poor outcomes - like people with obesity getting measurably worse care and skinny people being under-tested for heart disease and diabetes because those are "fat people diseases".
2) Not all weight loss is the same. Carefully constructed diet and appropriate exercise tailored to your goals under the supervision of a doctor? Great. Some pill from an unregulated supplement manufacturer? Not great.
As a professor of mine once put it, the swiftest way to drop your BMI is to suddenly have one leg and both arms amputated. But few of us would say that's healthy weight loss.
I think it's really easy to lose sight of just how many ads go beyond health and sell an idealized image in order to make money off of people's insecurities. Would you agree that there are probably at least a few ads that do that instead of promoting healthy practices?
But if a product is intended for weight loss, I would expect its ads to focus on weight loss...
I would investigate that possiblity before baldly asserting that the decision was guided by feelings and not facts. Just saying "but obesity is dangerous" should not give a free pass to deceptive/harmful weight-loss ads.
EDIT to complete the thought: If Pinterest did the analysis to determine that all or almost all weight-loss ads are deceptive or harmful, then just cutting them off is the right thing, and is based on facts, not emotion. I don't know if that's the case, but you need to know that piece before you can say for sure it was based on emotion.