He is constrained by the American people. However, that is his end goal.
Assuming his convictions were genuine in the first place and haven't changed, he is constrained by ideologues, venal politicians in the pay of insurance companies, and his own lack of balls.
Like Barbara Wagner in Oregon, the government-run healthcare plan would pay for the drugs to kill her, but not to keep her alive.
Many people with private insurance have found that their insurance didn't cover Tarceva, too -- and we're not even talking about people with no insurance. The Oregon Health Plan paid for 3 years of cancer treatment that would have been completely unavailable to Barbara Wagner under a free-market health system. The plan offered to pay for pain medication, hospice care, physician-assisted suicide, and other options. It declined to pay for Tarceva, a drug that buys a very small percentage of patients a very short interval of extra time. She didn't choose physician-assisted suicide and was not pressured to do so. She also didn't choose hospice care or any of the other options available to her under the plan.
Ms. Wagner obtained a promise of free year's supply of Tarceva from the manufacturer, which all the Tarceva anybody ever needs since it doesn't work for more than a year even in the happiest cases. Despite receiving the Tarceva, she died on 18 October 2008 - two months and twelve days after the date of the story you linked to.
Meanwhile, the Oregon Health Plan has been made to regret informing Barbara Wagner of the full range of options available to her, and has adopted a policy under which that will no longer occur.
