It can be both. I've spoken to my doctor about the "No new Medicare/Medicaid patients accepted" signs and he tells me that compensation is well below the cost of providing the care, so the only way they can provide ongoing care to their existing patients who are on or graduate to Medicare is to accept only privately insured new patients. Even this is no panacea; my new ACA-compliant BCBS-TN policy pays providers at a lower scale, and they do not accept new patients with that plan either.
This is a major problem and it's not just the ACA and Medicare/Medicaid. For years, private insurance companies have been negotiating reduced compensation for health care providers. We just had a doctor cancel an expansion and renovation project we had already finished. Between the cuts in Medicare/Medicaid compensation, the new people with ACA-compliant but low compensation plans, and the general reductions in compensation for general practitioners, he came to the conclusion that he was working for practically nothing, and if you are losing money on each patient, seeing more patients is no solution. Even with adding nurse practitioners, he could not develop a workable business plan which would give him enough space to add nurse practitioners and break even paying for the expansion. Thus instead of expanding his practice, he retired. Two of the GPs (one was in geriatrics which is nearly exclusively Medicare) at my doctor's office have retired for similar reasons, and honestly I think the only reason my doctor hasn't followed them is that he is one of the Doctors Without Borders - he regularly makes trips to Africa and his practice keeps his license and certifications current as well as helping fund his trips.