Because if you use swimming pool DE or non food grade DE, it will kill your chickens. They do eat it. It is good for gut worms, as well as keeping the smell down in your coop.
Just my wild guess here, but even if the DE in stall dri WAS food grade, the clay they mix in it would pretty much cancel that out. I'm thinking that if the clay in stall dri is like the cat litter clay, you wouldn't want animals to eat that... one of my dogs decided to "sample"(ugh) the kitty box contents and the cat litter turned to this nasty, stick-to-everything mud in her mouth and the vet told me that stuff can cause blockages in dogs. As if the fact that she ate cat poop wasn't bad enough!!!
Oh well, I guess I will give up on the DE and just spread lime like they did back in the day. I have been trying to find it for about 2 months, and it is getting to be more of a hassle than it is worth. There is some on websites and catalogs, but I dont give out my visa # over the phone or internet.
Plus the shipping can be ridiculous...I know I can't find food grade DE around here either. I use seven dust, stall dry, and wood shavings and so far that works great for my girls.
Horses are more sensive to things than chickens, if its made for horses it Ok for almost anything. I put a layer of stall dri under my wood shavings. My chickens do not eat it. stall dri is made to be put under shavings the animals mix it in with there dropings, spilled water (Which should be cleaned up when found). Its ment as a drying agent. also help keep smell down.
I use Stall Dri and love it. I use it the same way the others use DE. I sprinkle about a cup full into the bedding every week, turn it with a rake or pitch fork, or if the chickens do an awful lot of poo, more often, and let them stir it around when they do their thing. My coop is dry, and doesn't smell, and in the summer had no flies. And, most importantly, my chickens are healthy.
I made a phone call, and according to the company, it does contain food grade DE. I haven't noticed that my chickens go out of their way to eat it, but it's so fine when mixed into the bedding that I'm sure they don't see it as a food.
I've been using it since about May of last year, and have had no sick chickens or problems as of yet.
BTW, I had asked about this product, and a few others, as an alternative to the DE on this message board and received all kinds of warnings and dire predictions from the DE proponents about using anything but DE. Have talked to a few veterinarians, as well as some other people (farmers who have raised chickens for years and years) who have said that there is no evidence that DE is the best or only product you should use for your chickens. DE is not easy for all of us to obtain, and it's expensive as well. Add some shipping charges to it, and it's ridiculous. Don't beat yourself up if you can't use it, just use what other product works for you. Use your own common sense to determine what is safe and appropriate.