The thing with the biggest loser is probably the fact it is a crash diet.
I echo the PPs who mention the use of the word syns to describe foods. IMHO that is dangerous as nutrition categorically isn't a syn: without it we'd die. The other issue I have is the group weigh ins and the suggestion that going off plan has caused a stall or even a gain. Doesn't take in to account that as women our weight fluctuates a lot because of our cycles. For me weighing myself daily for some time has helped me recognise trends so I don't panic if I'm a bit heavier than expected the day after I've had a meal in a restaurant (sodium), in the run up to my AF, or around ovulation.
Calorie counting and small deficits, as well as getting out of the habit of cheat days, has done me wonders. I was bad in the run up to Christmas and I put on weight, but I reach my "crap food limit" much more quickly, and my weight gain is manageable; I don't go back to where I was before.
I don't always fully calorie count anymore, but when I cook I still weigh off the carbs to ensure I eat normal portion sizes of those. I'll have chocolate every day if I choose to, but within limits. Others always go "ooh, I really shouldn't", but hey, if it fits your calories then who cares?
That's the thing though. Food is so readily available that we have it regardless of whether we need it, and most people don't need to make a packet of 50p pasta stretch very far, so they just cook however much they dump in the pan. And we can't be seeing to waste food so we eat it.
So to come back to the original question whether SW is a cult: Well, cult is a big word. The main problem is that it doesn't teach people truths about food. Food is demonised through the term syns, people aren't educated about the true nutritional value of food and thus don't learn to develop long term healthy habits.