The decision was joined by Chief Justice Tani Cantil-Sakauye, and Justice Kathryn Werdegar and Gov. Brown's three appointees: Goodwin Liu, Mariano-Florentino Cuellar and Leondra Kruger.
In a dissent, Justice Ming Chin said the case sets "a precedent establishing whether that section [of the 2014 law] can function as a true reform to achieve its intended purpose, or if it is an empty shell — just another rule that can easily be evaded with a little imagination."
Justice Chin said the ruling would render the public review process "meaningless."
"Dramatically changing the sentencing laws -- by permitting early parole for some offenders, contrary to the detailed sentencing scheme currently in effect -- is not reasonably germane to changing the treatment of juvenile and youthful offenders in the criminal justice system," Chin wrote.
"This [majority decision] is really the continuing of a tradition of California judges really not liking to kick proposals before they get on the ballot," said Loyola Law School professor Jessica Levinson. She said judges prefer "to let the people decide once these proposals get to the ballot whether or not they're going to accept them."
Gov. Brown has appointed three of the seven justices. So, did politics play a role in the decision?
"I think you can't just ignore the fact that the governor said that this initiative proposal is very important to him," Levinson said.
But just being aware of the politics behind a case doesn't mean it impacts the ruling, Levinson said.
"This is not to say they didn't do their jobs," she said. "This is to say they may have acknowledged the political importance of what was happening."
In legal pleadings, attorneys for Brown said if the court did not allow his measure to piggy-back on the existing juvenile justice ballot measure, there wouldn't be time to get it before voters this November, a presidential election with a higher and presumably more liberal turnout.
So what's the potential impact on future ballot measure proposals going forward?
"It will be very easy to amend existing proposals," Levinson said. "It could be the case that the next proposal could be very conservative, and then we all have to live with this decision."
In other words, imagine proponents of a benign measure on, say, promoting recycling that gets changed after the public comment period to mandate any local recycling program be cost neutral, which could essentially gut most recycling efforts. Theoretically at least, proponents of that idea will be able to use this decision to protect their amendments from opponents, just as Jerry Brown did.