In LFSs, fish are often placed in unsuitable tank set-ups (e.g. overcrowded, undersized, with unsuitable tankmates, in unsuitable hardness of water, etc) but this is only a short-term measure as the fish are likely to be bought imminently and hopefully put in suitable conditions by those who have done their research on the species' requirements - and, often, staff will adjust tank set-ups on a daily basis depending on what's happening. For example, in the case of what you witnessed, it may be that the betta would have been ok with the neons as a short-term measure for a week or two (especially if he had entered their territory, as opposed to vice versa) or it may be that some of the neons had been consumed and therefore a tank re-arrangement was ultimately going to take place if he hadn't been sold when you bought him.
There are some cases of fish and situations being different from the norm e.g. a docile species of fish being territorial, a fish living a much longer life than the anticipated life expectancy, some bettas that will tolerate tankmates (especially if they enter another fish's territory, rather than they enter his - and yours will now consider his) in the short- or even long-term, some fish that make it through a fish-in cycle seemingly unscathed, but, when it's clear what the risks are and what the advice is from 'bona fide' sources, a responsible fishkeeper really ought to do what is right by the fish as opposed to whatever they wish for themselves. I was really keen on guppies and platies when I first looked at tropical fish in the LFS, then did my research and realised that they required much harder water than I could give them, and it took me a good few weeks/months to come to terms with eliminating them as possibilities (and re-orient my thinking towards which soft-water species might appeal to me which itself took a while to do).