Medicare is an expression of the federal government’s powers to tax and spend — powers which may have Constitutional limits, but none have been found by any court. No one is forced to do anything under Medicare besides pay those taxes.
With 51 votes in the Senate and 318 in the House, Obama could expand Medicare to everyone in the country with nothing the Supreme Court could have said about it. Obama didn’t have the votes, and Obamacare was the dodge.
It is a different issue, but Medicare has many rules, maybe not forcing people to use it, but aren’t doctors forced to take Medicare patients?