I wouldn't make that a cardinal rule, some LFS people actually know what they are talking about, keyword: some.
Chain stores really shouldn't be considered LFS.
I'd really like to see any documentation you have to prove this claim.
Ich is a parasite and it does not have any extended period of dormancy. If it's eradicated in its vulnerable life stage, or cannot find a host, it dies. All of the papers and information I've read related to it refers to it as an obligate parasite.
Since some fish can be carriers without showing any outward symptoms, or even have an immunity to the parasite, perhaps that's what gives rise to the idea that they pop up out of thin air.