Is it really ideal if the fish this person is purchasing are generations from this sort of water chemistry? I guess it all depends on the species of the fish, whether wild caught or tank bred...
But if this person is just going to the pet store and getting some neons which have been sitting in the shop tank with a pH of 7.4, and then quarantining them in a pH 6.5 tank, I don't see how that is actually doing the fish good, quite the opposite.
Cats are originally from the desert, I don't think most housecats feel more at home in the Mojave.
Evolution and domestication (genetically) did not take place overnight. Because they were kept and survived at 7.4 does not mean it is best for neons for an example. No different from keeping Mbuna in neutal water although it is best to keep them in condition you probably are aware of.
btw, not all the lfs/facilties keep them at 7.4.