No-one is saying that stress is not a factor. Many people think that genetics is a larger factor.
I've had this argument before, but here we go again...
When importing fish, the importer wants to sell as much of their stock as possible. If they have a beautiful strand, that almost no-one else has, then they are likely to sell their whole batch.
If those fish are fertile, and breed easily (as guppies tend to do), then repeat sales will be low (due to local sales of next generation fish). If those fish are infertile, or have offspring that do not match the parents, then the importer is going to have a better chance at selling their next batch.
I do not condone this behavior. I am merely pointing out that it happens. Not all importers do this, but some do.
As for sick fish, there are big quarantine hoops to jump through, especially with guppies, which are known to carry diseases. The importer is not trying to make his fish sick. That does not mean that they are perfectly healthy. He needs them to be healthy enough to get through quarantine.