I believe there may be two primary reasons behind laws like this:
1. They don't want the wild population hunted to extinction for the sake of the pet trade.
2. People are even more likely to release captive pets into the wild if the know their wild counterparts already live and thrive in the area. This is both cruel to the captive pets AND dangerous to the wild populations when they are potentially exposed to exotic germs and parasites picked up in the pet industry but not found in their native environments.
There may be some other reason...but those spring to mind right away.