hmm. It's a grey area as far as I know.
I mean there are so many things that could have happened wrong at your end. And I don't necessarily mean wrong but the conditions that you are keeping them in could be very different to what the store has them in, and therein lies the problem.
In the same way, you may have just got duds from the store, but on the other hand the fish could have been healthy at the store and just stressed with the move etc., and decided to give in with the added stress of new water etc.
Who knows really. When I get a new fish I put it in a bucket and add water from my tank into the bucket that has the water the fish came with. Acclimatise over the next 30 mins or so.
With some of my more expensive fish I have done this over a week!