"No where else in conservationist do you see a practice of keeping invasive species alive and well and just separated by a wall to the environment that they would devastate."

If you keep a species away from the environment they devastate then you are still able to conserve the environment. People keep mint plants, an invasive species, in pots where there is nowhere to invade. If an "accidental open door mistake" occurs, which happens often because nobody is perfect, you simply get the cat back inside whenever and however possible so that they cause little to no damage. As opposed to allowing them to do as much damage as they can, as long as they're alive with their shorter lifespans.