That is a fair question, and I do wonder how severe the issue actually is. I know I have seen a documentary that pointed the decrease in Desert Tortoises on the interchange between wild populations and released pets, so at least in that species the issue has been researched and documented. I also know that The European Pond turtle and some species of Asian Turtles have seen their numbers decrease due to the introduction of Red Eared sliders and the competition for food that they bring and the exposure to new pathogens. In general while not as well researched as one would like, there does seem to be some truth to it.