I don't doubt your specimens aren't in dirty water. I'm sure you're a conscientious, experienced fishkeeper.
The problem is that beginners see bettas in jars, assume that's the best way to keep them -- and it isn't, I think we can both agree on that -- and wonder why their betta dies in a few weeks. All bettas, and I mean ALL bettas, will do better kept in a heated, filtered aquarium. You may be able to keep yours just fine in jars, but a lot of people can't, and over at WWM I get "sick betta" messages very regularly, and it's always the same darn thing: saw the betta in a jar at Walmart, read that that's how they keep them in Asia, so why is my betta sick when I'm doing just the same thing?
Keeping bettas in jars may be the only economical way for experienced aquarists to hold large collections and raise their offspring to sellable size. I accept that.* But they are a terrible example for fishkeeping generally, and the reliance on salt to keep bettas healthy simply underlines the fact that the way they're being kept isn't optimal. End of story really.
So can we lay this thing down to rest now: salt has therapeutic value in certain situations, but there is NO reason to apply it routinely, and if you're keeping fish in a heated, filtered aquarium you should NEVER need to add salt except as a medication.
Cheers, Neale
*Though personally I don't like seeing bettas in jars and probably never will. I'm not a fan of fancy bettas for much the same reason I don't like munchkin cats or dogs like pugs with faces so pushed in they can't breathe properly. There's a line you cross when breeding "fancy" animals after which point the animal isn't really able to function like its ancestors, and my gut feeling is that fancy bettas crossed that line. Some people may think they're pretty, but I'll be brutally honest and say that I'd sooner a wild-type betta that could act normally in a community tank than a fancy betta that can barely swim.
None of my Bettas are ever in dirty filthy water.