It's because the market is not regulated. Pet stores try to sell it to you because it's how they make their money. And if your animal dies, you're probably going to buy another one. It's pretty sick when you think about it, but to them, these animals are nothing but profit. My animals are my babies, and I want the very best for them. All 21 of them.
I've never had any mold problems with the shelf liner, but if mold becomes a problem, you may want to remove the water dish because the humidity would be too high for a beardie at that point. Ideal humidity for a bearded dragon is 30% or lower. I live in Florida (very humid), so I don't use water bowls in my vivs so that I don't raise humidity too much. As it is, my humidity tends to hover between 28-35%. I measure my temps and humidity with the Acurite Digital Thermometer with probe. It gives me an accurate readout of the basking spot temp, the cool side temp, and the humidity, and you can buy them at Walmart for $12. Similar to this one
http://www.walmart.com/ip/Acu-Rite-Indoor-Outdoor-Thermometer/896347
It's actually this one, but I can't find it on the Walmart website.
http://www.acurite.com/thermometer-with-wired-probe.html
But they are really the same thing.