Anyhow, I fired off an E-mail to Costco's customer service desk, and here's what I got back:
Thank you for your email to Costco Wholesale.
Yes. As a condition of our alcoholic beverage licenses in California, all warehouses must permit non-members who wish to purchase alcoholic beverages to enter and make purchases. Non-members requesting entry for the purpose of purchasing alcoholic beverages should be directed to the membership department.
Costco Wholesale Corporation