I'm not aware of any general standards for distinguishing between abusive contracts and beneficial ones and I'm not sure there could be any. But the question "would anyone who really understood what was going on ever want this contract?" must be a relevant one. You could argue about how much humility should be applied: after all, just because I can't see any reason why a rational person would sign this contract doesn't mean there isn't one. So as is often the case, the optimal policy is not obvious. But make no mistake. The libertarian policy will result on a whole bunch of vulnerable people (almost none of whom will be libertarians by the way) getting screwed over and over and over again. You could (barely) argue that the libertarian policy has benefits that overbalances these costs, but there is no denying that the costs are real and big.