I guess my only comment is that even if being kept inside all the time is bad for cats (which is really yet to be seen, cats live much longer lives on average indoors, but I guess it depends on what you mean by "good" for cats) you still can't justify bringing an invasive species into an area where they're not meant to be and justify it by saying "it's bad for their health". Then let cats be outside where they're native. Let them be outdoors in Europe where they're not as much of a threat and prey is more adapted to them. But letting them be outside in places like the US because it might be better for their health while it's been proven to be bad for the health of hundreds of other species is silly, and I think anyone that is looking at it objectively can agree with that.