There are some folks that WW will not "allow" to join the program: children under the age of 10 and those folks who are not overweight--unless they lost weight with WW and are "at goal" and lifetime members, of course.
The "rule" is: "Weight Watchers prohibits membership for children under the age of 10, those individuals with an active medical diagnosis of bulimia nervosa or anorexia nervosa, those whose weight is less than five pounds above the minimum weight of the WW weight ranges, and during pregnancy."
As mentioned upthread, the weight range goes REALLY low--so to be at the low end (minimum weight for your height) would be pretty thin (and certainly not overweight enough to justify membership). For example, a 40 year old 5'4" woman would have to weigh at least 122 to join.
As for them "discouraging" fast losses, while the official literature educates you on what a healthy weight loss will be, and warns you about possible negative side effects of losing too much weight too fast--whether or not a leader would actually take you aside and schedule a meeting with you to make you stop or whatever would depend on the individual leader. Some would do that, some woudn't. I am sure most if not all would if they thought you were really making yourself sick.