> When cops "enforce" the law, like arresting someone or towing a car, they're only allowed to do it because there's some immediate need.
Not at all true. They can enforce the law because there's a law being violated, not because there's an immediate need for the enforcement.
> In the former case, it's because having a criminal roaming around the streets is a danger to society, and in the latter case because the car is blocking traffic and needs to be removed.
There are so many cases where cops can arrest someone who isn't being a danger to society in any way, like someone who illegally crossed the border into the US (a criminal misdemeanor) and is otherwise fully law-abiding. Or for an example under state law, a cop arresting someone who is intentionally underpaying state income tax (criminal tax evasion) has no immediate need to take that person into custody before conviction but is 100% allowed to do so if probable cause exists, at least until the initial bail hearing.
> In both cases you still need a judge to ruled that the person actually shoplifted or parked illegally.
Not before a cop gets involved, no. The judge comes after the cop.
> None of these factors apply in a dispute over pricing, and it's not the police's job to strongarm the shopkeeper to accept the lower-marked price. Indeed, in the two examples, there are often cases where no actions are taken at all, for instance issuing a summons instead of arresting someone, or issuing a ticket instead of towing a car.
This has nothing to do with strongarming the shopkeeper to accept a lower-marked price in the sense of an ordinary pricing dispute between private parties, it's about enforcing state or local laws that regulate this in cases where a shop is violating applicable laws.
It is true that many of these laws only allow administrative fines in response to complaints or inspections, not anything as proactive as I was describing. The theoretical viability of my idea of simply leaving with the item after paying the legal maximum price at the cash register and involving the cops if stopped actually depends on state contract law, and likely specifically its judicial precedents: if that state would view the buyer's offer to buy at the shelf price as accepted on the terms of the store's invitation to treat since the counteroffer from the cash register's scanner was illegal, then title transfers to the buyer at the time of payment and an attempt to stop them from leaving would be a crime that the cops could in theory be called for. If the state would view the buyer's offer to buy be rejected even though the counteroffer was itself illegal, then yeah the only available enforcement is the administrative complaint / inspection / fine procedure and the buyer never gains title to the property. I expect this legal conclusion would vary from one state to another.
I think we all agree that this theoretical option is very rarely practical, and I'm not pretending otherwise.