Costco's policy is that they cannot preform service on tires not purchased through Costco, I don't see why this should come as a surprise to you. As mentioned, there are members who HAVE purchased tires that would feel very strongly about having the tire center's already long wait times pushed back further for non-purchased service. Beyond that, by not servicing tires that were not purchased through Costco, it also reduced liability for the company. SHOULD something happen to your tire after having it serviced at Costco, they are now responsible for any damaged incurred.
A plug and/or patch typically isn't more than $10. There wasn't a Discount, Bell, WalMart or any various service centers nearby?