You’ve been brainwashed by Madison Avenue.
There is good store brand soda, and there is bad store brand soda. Sam’s cola isn’t bad, it’s just different (admittedly it’s not Royal Crown Draft, but then neither is Coke or Pepsi). I can live with most store brand colas, and I quickly learn which ones I can’t stand and don’t get them more than once.
I’m a lot more picky about root beer. I’ll generally pay the difference and get A&W or IBC, or occasionally I’ll splurge and get Henry Weinhard’s. And I’ve gotten to the point where I won’t even TRY store brand lemon-lime sodas; if it’s not Sprite or 7Up I don’t want any.
Besides, store brand sodas are sometimes fun because they come in bizarre flavors: Sam’s Raspberry, Hi-Vee Grapefruit, or one of the Shasta (I know it’s not really a store brand) flavors intended primarily for the Hispanic market: Mango or Tamarindo (sort of like thin, carbonated, A1 steak sauce… it tastes better than that sounds, though).