> me never going back there again is going to cost them a whole lot more in the long run, than them missing out on my membership fee from my occasional visit.

That’s actually not really true for Costco. They famously break even on selling merchandise as a way to sell the memberships. It’s unfortunate but the model really doesn’t care about occasional visitors like you.

Not excusing the attitude, that sounds completely unacceptable, but blocking shared memberships is core to their approach.