Because quite simply it's not digestible. It would be like a person eating glass. If you're lucky it might not kill you right away, but eventually it will. Calci-sand will clump up in a reptile's insides, and there it will stay, making it impossible to pass. How can they sell this? The reptile industry can make erroneous claims all they want to market and sell a product, it is not regulated at all.
I've been in a few good pet stores, mainly Mom & Pop type shops, but for the most part, they don't care about you, or your animal, all they care about is $$$$$$. Most of them don't know squat about the animals they are selling.
Take the calci-death back to the store and get your money refunded. Then use that money to buy non-stick shelf liner, reptile carpet, butcher's paper, or tile. Or, you can use newspaper or paper towels. These options are FAR safer, and will save you money and spare you misery, too.
-Nicky
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Loneliness it shadows me, quicker than darkness
Crawls to the surface of my skin, visibly surrounded by it
Black is all I feel, so this is how it feels to be free
-Alice In Chains

2.1 Bearded Dragons: Gordo, Layne, & Chiquita
2.4 Cats: Zeus, Remi, Kiki, Delilah, Sadie & Cheyenne
1.0 Betta: Mr. Purple