Exercise & Nutrition VS Slimming centres(part 1)

Throughout my 4.5years as a certified fitness trainer i've come across soooo many female clients that have at least tried "slimming salons" or "weight loss centres" once before embarking on a new lifestyle that embraces exercises and clean eating with me. From all the stories i heard from them(success or not), i came to a conclusion. All of the slimming centres require them to FOLLOW A STRICT MEAL PLAN and DISCOURAGES EXERCISES OF ALL SORTS. Why are this people not allowed to exercise? Isn't it suppose to help them lose weight faster?