In 2009, decades after Montgomery's sentence, the Supreme Court decided Miller v. Alabama, which rendered mandatory life without parole statutes unconstitutional insofar as they apply to juveniles. The decision was one of several decisions, starting with Roper v. Simmons, which incorporated insights from developmental psychology into criminal justice. We now know that the brain continues developing well into our mid-twenties, and that impulse control, resistance to peer pressure, and the ability to consider long-term goals are not quite there yet for juveniles. So, sentencing them to a lengthy period of time, without option to reconsider, now seems unjust in light of what we know of their cognitive capabilities.
But what about people who were sentenced to automatic life without parole before Miller v. Alabama? Thousands of these folks, who were teenagers when they were sentenced, are now middle-aged or even elderly (certainly by prison standards.) Many of them have spent most of their life in jail. Should their sentences be reconsidered? In other words, does Miller apply retroactively? This morning, the Supreme Court ruled that it does--and that states who used to apply these schemes to juveniles should now award them remedial parole hearings to reconsider their possible release.
The technical question at the heart of Montgomery has to do with the retroactivity rules. Imagine a situation in which a criminal justice rule is changed in a way that could benefit defendants.
Teague v. Lane, defendant no. 1, whose case hasn't even started yet, will of course benefit from the new rule, which applies prospectively. Defendant no. 2's case is still alive--that is, it's undergoing an appellate process or the time to appeal hasn't run out yet--and because the case is not "final" yet, she will also benefit from the rule change. But Defendant no. 3, whose case has already become final--which is to say, she exhausted her direct appeals, or the time to appeal has run out--will not be able to benefit from the rule change. There are two exceptions to this doctrine: the new rule will apply retroactively if it is either a "substantive rule of Constitutional Law", which includes “rules forbidding criminal punishment of certain primary conduct,” as well as “rules prohibiting a certain category of punishment for a class of defendants because of their status or offense” or a "watershed rule of criminal procedure", which is to say, a very momentous change (most rules are not that important; when SCOTUS thinks of such a rule, they think about things like the right to counsel and somesuch.)
According to the today's ruling, Teague v. Lane applies not only to federal defendants on habeas, but also to defendants like Montgomery, who have been using their state's collateral proceedings. Or, as Justice Kennedy stated for the majority,
The Court now holds that when a new substantive rule of constitutional law controls the outcome of a case, the Constitution requires state collateral review courts to give retroactive effect to that rule. Teague’s conclusion establishing the retroactivity of new substantive rules is best understood as resting upon constitutional premises. That constitutional command is, like all federal law, binding on state courts.The rule in Miller is, according to the majority, a "substantive rule of Constitutional Law", as it doesn't merely address process--it addresses the question whether a certain category of punishment (in this case, mandatory life without parole) is applicable to a certain category of people (in this case, juveniles.) It stems from the string of cases that recognized that children are different from adults in their “diminished culpability and greater prospects for reform.” These differences mean that imposing automatic LWOP on children “poses too great a risk of disproportionate punishment.” While the decision has a procedural component--the need to hold a hearing before imposing LWOP on juveniles--it is, in essence, a substantive statements that juveniles should simply not be subject to that category of punishment. The Court is not concerned about the extent to which this hinders finality--in this case, the state really can no longer exercise its punitive power by sending juveniles to prison for the rest of their lives without discretion, and once there is a social agreement there, finality arguments really do not apply.
The Court also gives states that used to have mandatory LWOP schemes for juveniles a corrective path: they should award people who are currently serving time under these schemes for crimes committed when they were juveniles an appropriate opportunity to be heard and to provide evidence for their rehabilitation, in which their young age at the time they committed the crime (and all we know about the implications of age to development) shall be taken into account.
Justices Scalia, Alito, and Thomas dissented, on the grounds that the Court had no jurisdiction to decide the case and that its classification of the Miller rule as a substantive rule was overbroad beyond the original intent in Teague.