To my surprise, sale periods in France are regulated by the government, meaning that companies can only have sales during specific periods decided on by authorities. The goal behind these regulations is to limit competition and to protect companies from having sale wars with their competitors. To the consumer, this regulation manifests in massive sales being held for limited periods throughout the year. The period of June 12th until 25th was one of those periods.
During this sale period, it seems like part of the regulation was that only loyalty cardholders could take advantages of the sales, so-called Ventes privées. The goal of this regulation seems very simple but also very clever; in order to benefit from the sale, customers have to sign up for the loyalty program if they are not a part of it already. By incentivising customers to give away their contact details, this promotion regulation aims to give companies the data necessary for future promotional mailings and thereby hopefully increasing future sales and boosting the national economy.