This is a tough issue, I feel a couple different ways about it because I can see the good and the bad in a lot of peoples views on the subject!
I agree that its not really a rescue if you buy the betta, there's no way around the fact that paying for the fish is giving the store money which is saying to them "Hey! It doesn't matter how badly we take care of our fish because people will still pay for them!" And that's not a message that you really want to give out, I consider paying for a "rescue" a sympathy purchase.
But I also agree that no matter how you end up with the fish: Buy it, demand it for free, steal it (Bad idea, don't do it!!) you're still making room for another fish to take its place.
Most stores, at least the big ones like walmart, petco, petsmart ect. don't lose money on dead fish, they take a tally a couple times a day of the fish that have died and at the end of every week they send it to the supplier who reimburses them for the dead fish.
So what should a person do? Well getting the fish for free sends a message, not all stores will give you a betta, even a dying one, for free, but some will if they don't think it will survive any way, and that they do lose money on.
But I feel the best thing to do is try and get them to change their ways, don't take all the fish away, find a way to get the fish better taken care of in the store, its not easy, trust me but it is possible!!
I absolutely agree take better care of fish and every other animal for that matter.
Walmart doesn't sell fish in my area but I have the other big boxes. I hardly think they are going to miss my 2.79 sale- they charge a ton for aquariums, food, decor, etc...
I'm lucky to have a few good LFS's in my area but for those who don't- are people really going to pay 35-40 dollars to ship 10 bucks worth of fish.
This whole debate is multI faceted really when you look at from an animal loving point of view.