The supreme court is not bound by stare decisis. Particlualry in regard to questions of constitutionality.
Indeed stare decisis should not be used by them in determining the constitutionality of a law, particularly if a justice is of the belief that the court has previously ruled incorrectly on the same issue.
They may change their minds as often as they desire or are so compelled by their adherence to the constitution.
Any justice who decides the constitutionality of a law based on stare decisis and not on the constitution is, IMO, not worthy of sitting on the bench.
The justices are compelled to uphold and defend the Constitution as they believe it to mean. Any adherence to stare decisis when they have questions about whether the court was wrong in the past is a violation of their oath.
It's not absolute, but it is critical. One must be cautious in overruling old precedents. Remember the old Roscoe Pound line, "The law should be stable, but never standing still"? It has to be able to evolve, adapt, and learn, but it can't just change willy-nilly. Starae decisis is critical for that - and that's why mainstream jurists - like it looks like Roberts is shaping up to be - have a deep respect for precedent, even while remaining mindful of its limitations.