Ich is found on a fish because the fish/environment has been compromised in some way recently, lowering the fish's immune system and resistance to bacteria and illnesses.
Ich is not on a fish because it is a healthy fish in a stable, unchanged, uncompromised environment.
Ich is almost always present in all freshwater aquarium water (especially LFS's). Take a stressed fish that's just been caught and bagged for transport and expose it to a rapid change in water temperature....the ich will more than likely show up on that fish real soon.
No, no, no.
Ich is a parasite that can be easily eliminated from aquaria with a little knowledge of it's life cycle.
There is a small chance that, if treated improperly, you can reduce your ich infestation to such a low level that the fish display no symptoms yet are ripe for an outbreak.
However, if instead the ich parasite is properly killed, using the very simple salt and heat method, it WILL NOT BE IN YOUR AQUARIUM and no amount of mistreatment of the fish will cause them to get ich again.
I am with you on one point...you can pretty much just KNOW that new fish (or plants even) from the LFS
WILL likely come with some ICH parasites and you might as well treat them in quarantine the moment you get them home, white spots or not.
Your mistake was the phrase
"almost always present in all freshwater aquarium water," which seems to be saying that ich is some kind of mystical airborne pathogen that we can't do anything about except try to keep our fish comfortable most of the time. No, ICH can be easily completely removed from any tank. I guarantee you with 100% certainty and my home up for collateral, that none of my tanks or fish can be caused to get ICH right now, no matter what you do...because there is no ICH in them.