Why do doctors and nutritionists slam crash diets yet reccomend gastric byp surgery?

I hear all the time about how health professionals don't want us to go on crash diets because they're unhealthy, but these same doctors perform and encourage morbidly obese people to have gastric byp surgery, where your stomach is reduced to the size of a walnut? People who have this surgery can't eat more than an ounce of food at a time for several weeks. Why is it OK to starve after surgery, but not on your own? There is no way an extremely obese person is going to be able to change their eating habits so they can lose 1-2 lbs per week, which is what most nutritionists reccomend.