Well, being a colleague of the Pets at Home company I see this a lot - especially as I try to float around the fish section as much as possible.
In the companies defense they are one of the best "chain" stores as they actually provide training for their staff, this training may be basic but it covers the main starter areas of fish keeping, along with small animals, dogs, cats etc.
It is down to the individual colleague to then take it upon themselves to learn more about the section if they so desire. If that colleague has no interest in fish, they will not waste their time in doing so (unlike myself who has a genuine interest and couldn't care less about the small fluffy things). Along with daily cleaning of the rabbits, guinea pigs, reptiles etc there are hundreds of other tasks that require doing and the fish normally don't get enough attention because there just is no time.
It's down to you as a customer to realise this - I personally would rather visit a purely aquatic shop where they have genuine interest rather than somewhere that does everything.
Now I work my backside off (not now as on maternity) in my fish department, fighting against a variety of higher authorities on not selling fish for particular reasons so yes, as stated above, please do not make this a general over view of the company as a whole.
I suggest you take your white fish receipt form you were given when you purchased your fish back to the store, if possible, bring a water sample for them to test and even better, pictures of the loss of fish. Pets at Home are more relaxed than most on their refund policy and I'm sure if you speak to the manager there will be no problem as long as your tank is fully cycled and adequately sized for the stock.
The white fish paper work is your proof that they should have gone through the basic questions and made sure your tank was suitable. There is also a "health" box on there where the colleague must check the stocking before selling it to you.
Anything else you need to know, please do ask.