It was these poor bettas lucky day when you walked into the store! Those ammonia readings were disgusting!! I see this sort of thing all of the time here in Australia. The largest LFS here constantly has dead bettas in their tanks and the water is usually so dirty that you literally cannot tell what colour the betta is or whether it is even still alive.
I have tried on countless occasions to politely suggest that they really do need to clean their betta tanks more often, or bring it to their attention that they have sick fish and "this is what is wrong with them" "this is what caused it" and "this is how to treat it" they just treat me like I'm stupid and don't know anything.
Our smaller LFS's on the other hand even ask for my advice when they get sick fish. One had an outbreak of velvet with around 50 fancy bettas affected. She was quite upset that her poor fish were sick and came rushing over the minute I walked through the door to ask for my opinion on them. Two weeks later and all except one recovered fully. One of the LFS even gave me a job! It is important to remember that some LFS do care, not in all cases but I usually find that it is the large chain stores that are the culprits.
Surely stores such as Petco will have to realise at some point that keeping their bettas healthy by providing the simple care that they need is not only morally rewarding but financially too!!!!!!!!! Seriously do not know what it is going to take for the light bulb to light up though