That's not how it always happens. In fact, I'd venture to guess that the majority of people use credit with every intention of paying it back. But sometimes **** happens. Sometimes, significant others run up your credit. Sometimes people promise you things and you make decisions based on that, and they don't follow through. Sometimes, you use credit and then lose your job. Sometimes, you get divorced and all the hassel and expense that can come along with that. Sometimes unexpected expenses arise. (such as medical, as you mentioned... but sometimes it's home repairs, or vehicle repairs, etc) It's VERY easy to find oneself in a hole you can't get out of even though you were doing everything you could to do the 'right' things.
I just find it a very 'holier than thou', self-righteous attitude to assume that everyone who has a bad credit history is there because they're bad people. It's been 10 years since my bankrupcy and I've worked my ass off to pay back the debt, limit current debt, and make sure that every payment is there on time and nothing is missed, and my credit isn't checked too often (because every check makes my score go down). After 10 years of keeping my nose clean, making very good money and doing the best I can, I STILL get turned down for credit. Sometimes payments are late not because I chose to send them late, but due to other factors - like not getting the bill at all or not getting it in time due to the fact that I move every 6 months or so. Or, setting up a payment schedule with my bank, but not realizing that the payment due date actually fluctuates a bit and after 6 months, suddenly my payments are being sent late - inadvertantly.
Not to mention the fact that even if people DID screw up royally and **** up their credit by doing something "bad", that doesn't mean they aren't legitimately trying to fix things. People make mistakes, they shouldn't be held to the fire for them for the rest of their lives.