In a lot of places in the US, the lower of the shelf price and the scanner price is by law the most they can demand, at least for retail sales to consumers. Attempts to stop the customer from leaving after having paid the legally appropriate amount would be criminal acts by the store, no?

Not to mention that cops only have powers to arrest/issue tickets, not to adjudicate disputes. This isn't Judge Dredd where cops can mete out judgements as they see fit. That's the whole reason why we have courts and judges.