I think the logic here is, "It could get worse as the tire flexes under load."
If your choice is between a 100% chance of replacing one tire now, or an unknown chance of replacing four tires (three of which might still have a lot of wear left), you might want to take a 100% chance of buying one tire early instead of a __% chance of buying four tires early.
Glad your tire shop says it doesn't need immediate replacement, @kyunam, but (even if safe now), you might take the gamble above into account for both safety and financial reasons.
Thanks for educating all of us! I've certainly learned from this thread!