Weight Watchers (WTW) - Get Report probably thought they were being proactive about childhood obesity. The internet let them know they are not. 

Weight Watchers announced that it will offer free memberships to teens 13-17 this summer as it looks to help "the development of healthy habits at a critical life stage."

The National Eating Disorders Association said it was "very concerned" about the promotion because 35% of dieters can develop disordered eating and teens are especially vulnerable to such disorders. 

"We hope Weight Watchers acknowledges the risk and implements steps to screen for potential early signs of disordered eating and provides information on resources to find help," the NEDA said in a statement. 

Other health advocates said that encouraging teens to count calories and diet is dangerous. 

"Our decision to open our program to teens, with the consent of a parent or guardian, is driven by a family-based approach," Weight Watchers said in a statement to CNBC.