I've discussed this cordially and at length with the regional manage of Petco (who assures me that Petsmart has a similar policy). They are not authorized to discount fish, especially if it appears unhealthy. They get reimbursed if the fish dies.
They won't sell sick fish in the event it infects your tank and spreads disease to other livestock. The fact that this makes no sense when applied to Betta is not a consideration for hide-bound, inflexible, bottom-line, beancounting corporate types.
If you like the fish, buy it. Fish are cheap. If it dies within the warranty period, you'll get another fish or get credit.
Don't humiliate yourself and get a bad reputation by wheedling and whining. But do write a scathing letter to corporate HQ complaining about conditions. And DO mention the manager by name.