I was in a local Dickey Bub True Value last year and saw them spraying something on the feed. When I asked what it was, he said bug spray to get rid of the moths. I have never bought feed there since then. And they did carry commercial feed.
I have to think it was more likely local store ignorance that killed your flock, not commercial feed. Commercial feeders wouldn't be able to stay in business if they poisoned their feed.
However, I know there are exceptions. I was one of the unlucky cat owners whose beloved Momo got kidney disease from melamine poisoning in Chinese-made cat food in 2007. And Mazuri (which I do rely on for my duckling feed) had a major recall in 2012 for excessive vitamin D in their formulas.