Justice Powell, joined by Justices Brennan, White, and Stevens, agreed that South Dakota could reserve cement needed for the State's own public projects. But he rejected the majority's rule that a State may withhold a publicly produced commodity from interstate commerce simply to advantage private residents and businesses within the State.
In his view, the relevant distinction was not merely between market regulation and market participation. The Court should instead ask whether the State was performing a traditional governmental function, such as supplying its own governmental needs, or operating a commercial enterprise in the private market. A State may make purchasing and supply decisions for government operations, but it may not use a state-run business to evade the Commerce Clause's prohibition on economic Balkanization.
Alexandria Scrap did not control because Maryland's bounty program affected market incentives through a subsidy rather than erecting a direct trade barrier. Maryland paid money that made in-state processing more attractive, but it did not prohibit the movement of junk cars out of the State. South Dakota, by contrast, directly cut off cement sales to out-of-state customers during a shortage.
The resident-first policy produced the same effect as a statute ordering private cement producers to reserve their output for South Dakota buyers—an effect the Commerce Clause would plainly forbid. Political pressure on a state enterprise makes it especially unrealistic, Powell reasoned, to equate the State with a private seller exercising ordinary business discretion.
The majority's manufactured-product distinction was also unpersuasive. A state cutoff of cement can burden interstate buyers just as severely as restrictions on natural gas, agricultural goods, or other resources. South Dakota's regional access to cement gave it the practical power to deny neighboring consumers the benefits of interstate competition.
Powell warned that the majority supplied no workable limiting principle. Under its approach, a State could manufacture food, pharmaceuticals, energy products, or other commercial goods and reserve them for local private interests. That result, he concluded, conflicted with the Commerce Clause's central purpose of preserving a national economic union.