Experts have said that the conservative-majority US Supreme Court has seen its reputation as an independent final arbiter restored thanks to its ruling on Obamacare.
Roberts silenced critics who believed that the court would vote along partisan political lines and strike down President Barack Obama's signature domestic achievement barely four months before his re-election bid is decided.
Along with the four liberal-leaning members of the bench, the chief justice's vote gave the Democratic leader's health reforms a 5-4 victory -- despite five of the judges having been nominated by Republican presidents.
At a time when three in four Americans believe the court's rulings are political, this ruling "shows that the chief justice cares deeply about the court's reputation," Fordham University School of Law associate professor Clare Huntington told AFP.
Only one in eight Americans said in early June that the court would rule on Obama's law based solely on legal arguments, according to a survey by The New York Times and CBS.
"What they're afraid of is that the subjective impulse of the judge, rather than something more objective, will control the decision," Justice Stephen Breyer, a liberal member of the bench, said in a recent interview.
The pivotal "Bush v Gore" ruling that favored the Republican candidate and saw George W. Bush become president in 2000 rather than Democrat Al Gore was hard for some liberals to swallow.
The highest court in the land was openly charged with bias when, in January 2010, it allowed the limitless financial participation of companies to political campaigns.
"The American people would have come to see the US Supreme Court as pursuing a far right-wing, pro-corporate agenda if it overturned the health care law," Karen Wagner, an Illinois resident, wrote in a letter to USA Today.
Roberts' "deciding vote saved the Supreme Court from itself," she said.
"I've always thought of the Supreme Court as having a kind of internal, institutional gyroscope that keeps it from veering too far out of kilter," wrote opinion writer Ruth Marcus in The Washington Post.
For constitutional lawyer Elizabeth Papez, Thursday's decision "avoids a direct confrontation with the political branches by upholding a contested political act. But it does so on legal grounds that unambiguously affirm the court's power to say what the law is."
Roberts leaned on the lesson of an early predecessor, John Marshall, who in 1803 also took a stance that reaffirmed the court's authority as the foremost interpreter of the Constitution.
Reading the ruling, Roberts declared: "It is not our job to protect people from the consequences of their political choices," referring to the court's "reticence to invalidate the acts of the nation's elected leaders."
"Most important for his place in history, he avoids having the 'Roberts Court' be the first court since 1937 to strike down a major piece of economic legislation," American University law professor Daniel Marcus said.
Papez said the decision "reaffirms the court's role in our system of government, and could be viewed as taking the same approach to preserving the court's institutional authority and credibility."
The court avoided "the damning critique of history," said Fordham law professor Abner Greene.