Seems like a case of due diligence on the part of the retailer.
The question the retailer asks is this: "Why would anyone buy so many gift cards with such regularity?"
Given that credit card companies may make the retailer responsible for unauthorized credit card purchases, this seems like an instance where the store manager wanted to cover themselves and not have their employer holding the bag for a pattern of potentially shady transactions.
Having worked in retail once before, sometimes the retailers do not want to confront potential scammers directly for fear of being seen as harassment and (nowadays) subject of a discrimination lawsuit. By calling the police with a pattern of odd behavior (compared to their usual customers), the retailer can show that they have acted with due diligence in case some party wished to pin liability on them for something.
Of course not justifying police detainment, just shedding light to an instance where retailers might call the police. Similar to the recent case of a coin shop where someone compromised another's bank account and wired the money the shop, seemingly legitimately. Due diligence worked out in that case for sure.