Some people consider kittens as cute and adorable. So for QPWS and RSPCA to allow freezing of the kittens, people would cause an uproar. Gas just puts them to sleep and is deemed much nicer. Not so many people care about the welfare of toads, however.
A gas chamber need only be a glass aquarium, CO2 is heavier than air and will accumulate at the bottom of such a container. Just make sure it is properly primed first.
As an aside, when this first blew up a few months ago a girl I know heard it on the news. She is actually petrified of snakes but easliy accepted using kittens as a food source providing the prey was humanly killed first. Needless to say the news article didn't mention that part.