Medicare is a subsidy the govt pays for seniors, Obamacare is going to cut medicare payments to doctors to try and control costs. So what does the doctor do? He refuses medicare patients because if a private insurer pays you $200 for a procedure, and medicare says "you only need to make $50" the doctor will choose private insurers.

Its the doctors choice to choose what customers aka patients he wants to see. If their insurance doesnt pay his prices, they dont get care and they can go to someone "cheaper".

Welcome to ECON 101

Of the $716 billion in cuts, $415 billion come in the form of “updates to fee-for-service payment rates,” a euphemism for reducing Medicare’s payments to doctors and hospitals. But what happens when you reduce payments to doctors? Doctors stop being willing to see Medicare patients. And if you can’t actually get a doctor’s appointment, what does it really matter what your insurance plan covers on paper?