The virus only survives on surfaces for a very short amount of time. Like on the order of a couple of days, if that. Even if an infected bird pooped in the grain field, it would be a LONG time until the contaminated grain made it into your chicken feeder - harvesting from the field, riding on a truck to the processing plant (or any distribution hubs), being processed into feed, being packaged, riding on another truck to the distributor/seller, riding on yet another truck to your local store, sitting on the shelf waiting for you to buy it, then riding back home in your car... By the time it reaches your chickens, there will be no avian flu left on it that's still alive and dangerous.Plus consider the grain has to travel, has been in a field, etc.
As for workers contaminating it along any point of this process, they can still do that even if they don't own chickens. Maybe the lady at your local feed store walked her dog out in the woods the morning before going to work, didn't do a foot bath and tracked wild bird poop all over the store. Which could happen at the grocery store or the convenience store or any other place just as well. There's no way to eliminate all potential traces. We can just do our best to address and avoid the highest risk activities (like walking our own dog in the woods with the same shoes we go into the coop with), and hope for the best.