I understand your concern 100%. My husband and I made the decision years ago to never purchase cup bettas from any chain store that kept them in poor conditions (and that purchases them from mills), which includes most chain pet stores. We made this decision for exactly the reasons you listed above.
Sometimes it is excruciating to see them suffer and know that you could potentially save one life. Ultimately though it is imperative to remember that potentially saving one life means dooming countless others. It's tempting to tell yourself "How much am I really impacting the world? What difference am I going to make?" But it takes the strength and resolve of each individual to stand up against abuse and "mills" in order to make a difference long term for the well being of these bettas. It sucks
. I don't allow myself to go to the fish section of those kinds of pet stores, because it is too heartbreaking to stand.
When you're in this situation, don't think you're helpless. Write an email or a letter to that store's corporate office. Let them know that you appreciate their store but you are abhorred by the lack of care that employees are taking. Phrase it in a way that the store sees that this is affecting their bottom dollar. The bettas are dying in their cups and not being purchased. People are being turned off by the poor keeping and refusing to purchase from their store. Etc. Let them know that you aren't against the selling of bettas, but of the employees not doing their job.
Another step to take is to educate your peers. I stopped a friend of mine from purchasing a pet store betta after explaining my views on the subject. She agreed, and is now researching betta care and needs, as well as local breeders in her area.
There are many people who will save that suffering cup betta, and I honestly cannot blame them. Not in the least. But I do agree with your side of the issue, and I want to tell you that I think you are doing the right thing.