I probably wouldn't buy more than $200 of stuff per year from them. So a membership would be quite expensive and not worth it. I see nothing unethical at all about doing the gift card thing if I am not going to shop there often, they at least get the interest free loan from me as well as chance I could lose the card.
I have a good local discount grocer for all my meats, which have much better deals than any big name chain at 30-60% cheaper. Beats Sams, Walmart, Aldi, Costco etc.. I shop at Aldi, Walmart and Sams depending on what has the best price. I keep a price book. Costco would just be for a few select items which happen to be a better value than all the other stores I shop at.
If they didn't offer this gift card route, then they would simply not get ANY shopping from me, no interest free loan for gift card and no opportunity for me to lose their gift cards to profit off of me that way. So by me "exploiting" their system, it only would benefit us both.