I believe they're required to do that, or that liability issues force them to do that. Every diet commercial on TV says that, too, as well as magazine ads when they show someone who has lost a lot of weight. It's been statistically shown that most people fail at diets, or fail to keep the weight off. Weight Watchers is one of the few programs that can claim a higher percentage of people succeeding and keeping it off, but it's still a minority of people.
Don't use that as an excuse to quit! Tell yourself that you will be one of the winners!

A good book to read is "Thin for Life" by, I think, Anne Fletcher. She's interviewed people who have lost weight and successfully kept it off for years.