You'd be surprised what they do to Marine recruits (and Army ones, too)
While hanging from a doorframe isn't part of the treatment, there are a lot of worse things they do. One of the purposes of basic training is to weed out the weaklings. If someone can't handle the pressure of training, they'd never handle combat.
If you get The Military Channel (formerly the Discovery Wings channel), you can learn what trainees endure. Odds are, you'd be surprised. Most people have no idea because they have no contact with the military, often not even knowing anyone who has served in the last 30 years.