A lot of pet stores just don't have the education. If the products are made, why shouldn't they be safe? Of course this isn't the truth, as many products which are unsafe are made. Hamsters do love the cotton bedding, so it's easy to see why they think using it would be good. "They love being snuggled up, it helps them make a nest". I thought it was good when I was 8 (over 15 years ago) and used it all the time with my hamster. We never had problems with it actually.
The lady I adopted our latest hamster from, Coffee, who's a 2yr old dwarf came with cotton bedding. She is a pet health specialist at a really great, I think, small pet business. Even still she had no idea. A lot of these employees know a lot about dogs, or cats. Maybe fish or birds. There is very little expertise in small critters, especially up here, and a lot of them don't really care.
Their managers, who probably do love animals, but may just be there for a job, have been told that this product is good for hamsters. The bags says that it's good. I can see why they don't understand.
It would take an entire product recall, or huge public outcry to stop it. And unfortunately with how spread out even our population is here, I don't see that happening. I'd love to see our pet stores not use mesh wheels, or not sell crittertrail or habitrail cages... but they do anyways. They're appealing for younger kids and they make money off them.
Unfortunate, but there's not much you can do. Good on you for telling them about the dangers, but a lot of people will stick their noses to it and go with what "they know" which isn't always right.
It's a pretty lose-lose situation, but that's how I see it. You just have to hope that the adoptees of the hamsters will do more research than those selling.