alleged injured party vs a states rights issue. Yes states and other entities (including private corporations) were the plaintiff's petitioners, but I think you are confusing the petitioners with wither this is a constitutional question of state vs federal power. The question in this case was not whether the authority to regulate CO2 emissions is a power given to congress in the constitution or reserved to the states. This is whether congress gave the EPA the authority necessary to issue some of the the regulations it did under the Clean Power Plan. Its all federal authority and authority that congress possess. The questions is whether congress then gave that authority to the EPA.

I am not sure what you mean that "this is a policy within its mandate." What mandate are you talking about? The Clean Air Act is the statute the EPA used as a basis for the regulation, so that is the mandate from Congress they are basing the regulation on. The EPA specifically used 42 U.S.C. § 7411(d) as the basis for this regulation. Clearly congress just didn't authorize the EPA to just clean the air, or something like that. There are specific statutory provision, that lay out what Congress is authorizing the EPA to do. That is their mandate here - at least that is what the EPA itself said was its mandate.

When an executive agency goes beyond what it is authorized to do by congress, it should be stopped. You disagree with the court that the agency overreached here and you might be right that the court will be less deferential to agencies in broadly interpreted what they have been authorized to do by congress. Ultimately, that is sometime easily remedies by a functioning congress - where we elected our representatives any they give authorization to expert agencies to undertake regulation. This is a line drawing question and you may well be right that the court is drawing the line to more narrowly interpret agency authority - but that does not mean that the agencies have no authority. Its the dysfunctional Congress that is the real issue - because they could easily allow the EPA to do this if they wanted to even under this court's ruling.

Who benefits from this? Well clearly the states and petitioners who brought the case think they will benefit. Frankly, if these kinds of decisions kick congress into gear and our elected representatives start governing instead of only fighting and looking to the executive branch to get things done based on who the executive is, then we probably all benefit.