"Thinking for himself" is not enough. Lots of drunk people "think for themselves".
Adult patient-
1) Patient must understand the gravity of their condition
2) Patient must be aware of treatment options, benefits and risks
3) Patient must fully comprehend that no treatment = death
If all of the above have been met, and patient still refuses care, then yes, docs should stand down. Problem is, the mere fact that one would deny treatment and accept death is reason enough for most to call in a Psych consult and the Ethics Team. Then things get muddy. Family gets involved, durable power of attorney gets thrown around.
Right now it may seem like anyone who would refuse treatment is foolish. That likely means that you have great faith in physician's interventions, and that you have been healthy most of your life. Now put yourself in someone else's shoes. Like the family whose mom died on the OR table during a great toe removal. Or the guy with CHF, Lung CA and renal failure on dialysis. If I were him, and I developed pneumonia, death really doesn't sound so bad.
Each person should be allowed control of their own destiny. Death is part of that. Our job as physicians is to act as a shepherd to those that need us. We're not to determine the fate of others against their will.