IMO what it comes down to is this... (note it doesn't just apply to fish either, but all animals being poorly treated in pet stores!)
A sympathy purchase every now and then is not a bad thing. You are making a huge difference in the life of that fish. It is always best to try and get the animal for free or at a reduced rate. Sometimes neither work and your heart wont let you leave the animal behind. If you are doing a sympathy purchase try to make it very clear to the store manager that you are saving the life of the animal and you do NOT agree with the conditions. Say you are going to complain to corporate and then follow through with said complaint. At least you made an effort to help the fish left behind by doing so.
The problem is when people just buy all the sick/cruelly treated fish they can get their hands on and call them rescues. In doing so you are being part of the problem, not part of the solution. You are simply opening space for more fish to fill, causing them to suffer the same fate. The store will not change because they have no reason to. They are able to sell dying and sick fish at full price. You are also encouraging other people to do the same thing, which causes a big boom in business when enough people do it because lets face it... rescuing animals feels good.
I hope this helps the members who do not understand why it is important to differentiate the two terms.
Erika | Betta Fish Advocate