You didn't quarantine the fish before adding it.
It's not the LFS's fault. If anything, it's yours.
In addition to that, the guy gave you free meds, which are expensive, usually more-so than fish.
That is utter crap.
If you are suggesting that for every fish you receive you should be placing them in quarantine ? Not everyone can afford to buy an extra tank that for the most time will sit there idle, not to mention the time and effort to cycle and keep it maintained.
Our lfs has a duty of care to the customer which includes the proper handling and care for goods that are sold. We know that not all lfs quarentine their fish so it is important to inspect the products prior to purchase for any signs of damage or disease.
If a fish dies or developes a disease (normally within 7 days) are refund is not normally disputed. In this case i think the shopkeeper acted responsibily and gave free meds to treat the problem which in most cases, as already stated, cost more than the fish itself.
It is nice to be in a position to quarantine your fish but most people only have the one tank, so to suggest it was the OP's fault is senseless.