Judicial activism against the environment
The background is that California law since Proposition 13 has allowed a simple majority of voters to decrease tax rates but requires a two-thirds supermajority to increase "special taxes" (taxes not just meant for general public benefit purposes). A "special assessment," however, is different from a "special tax" in that it provides a special benefit to specific properties, and doesn't require a two-thirds vote. The entire legal battle is over what constitutes a "special assessment."
Proposition 218, passed in 1996, tightened the legal requirements for special assessments in ways that remained poorly defined (probably to keep voters from anticipating specific problems and voting against the measure). As the Supreme Court notes, Prop. 218 created a paradox whereby a "special district" must constitute all the parcels that receive a special benefit from the assessment, but it also says that if the benefit reaches every parcel in the district then it isn't a "special benefit" but rather a general benefit that needs a supermajority to pass.
The Supreme Court dealt with this paradox by saying that if parcels receive "direct advantages" from the benefit, like proximity to a park, then it's a special benefit even if conferred on all parcels in the district. So far, so good. But then the Court spends the remainder of the opinion ignoring what it just said. The Open Space Authority had justified the assessment by describing the direct advantages all parcels in the district will receive, such as better views and improved access to public recreation. The Court ignores this and says because these benefits reach everyone, they aren't special benefits and fall under Prop. 218 proscriptions.
So why would the Supreme Court behave so strangely? The likely reason is that Prop. 218 was itself a voter reaction to a previous Supreme Court ruling that allowed expansive uses of special districts. The Court is aware that initiative propositions are the only mechanisms that voters have to overrule it, so the Court felt obligated to overreact to the Open Space Authority's special assessment, something that resembled a standard property assessment.
Short of changing Propositions 218 or 13, this decision is the last word on this issue, and an assessment like the Open Space Authority's will require a two-thirds supermajority. However, somewhat different special assessments may still be possible. An assessment that set up a priority acquisition area and assessed more in the immediate vicinity of that area than far away from there, or assessed more when an acquisition is first made, might still be legal. This is something that will have to be tested in the future.