Because...these snails reproduce fast. They can eat the plants other animals need to survive. Many animals in pet stores are not native to the country that they are sold in. If you release them, they can take over just like honeysuckle does now. They will become an invasive species who can outproduce and overpower their native "neighbors".
I don't watch Animal Planet, except for very rarely, and I would assume that they are not releasing things like snails, fish and bearded dragons into the wild. They are probably releasing things like, say, an orphaned hippo that was raised with minimal human contact and is native to that area. If they were releasing, say, a frog, it might be a rare species that was bred in captivity and released. They were bred in a habitat as natural as possible with, again, minimal human contact (I read about this in a National Geographic). I could go on and on, but I won't. Then again, this is a TV show. You can't believe everything you see on TV. Those animals that they were "releasing" could have been raised by humans for the specific purpose of being "animal actors". After filming they would be caught again and probably would be used for other things.
Atlas blue metallic marble HM
Forest red VT
Leander red/pink dragon Delta
Mister Intelligent orange/white BF VT
Phrixus teal marble dragon HMPK
Psyche blue/red PK
Rhea black/red BF DT
RIP Alexander, Morpheus and Phobos. You are missed :'(