I'm not sure I would call it poisoning, because the Carbon Dioxide isn't killing you. The lack of Oxygen is. Carbon Monoxide, however, will pretend to be Oxygen and block the hemoglobin from binding with Oxygen. That to me sounds more like a poison, because the substance itself kills you, no matter how much Oxygen is present. There is no danger to the snake of the Carbon Dioxide. It itself is not poison. You could use Nitrogen or something to the same effect. What matters is the atmospheric partial pressure of the Carbon Dioxide within the gas chamber. There is so much in the air, it prevents any in the bloodstream from coming out. Like how 100% humidity will slow down the evaporation of a bowl of water.

We're not killing them for fun (I hope). I've seen some feedings that would make you think otherwise, though. This one guy grabs the rat with the hemostats, and gets the tail twice (it looped) and the thing freaked out! It was thrashing and flipping, slamming itself off the workbench/rack. The guy laughed at it.

If you're concerned about it, you probably don't need to be.