I think he's saying that we're already succeptible to moral hazard in the same way that we would be if we colonized mars. People in America don't have much skin in the game when it comes to protecting the environment in India or China.
Except, we still do care if their water is toxic and if the air conditions are poor. The same sort of empathy will apply to people on other planets as well. Even though a heavily polluted other planet doesn't directly affect us in the same way that a heavily polluted Earth affects us, we are currently pretty well isolated from the extreme conditions in Shanghai and yet we still care what happens there.
Furthermore, because of the large costs of shipping between planets, most of the things causing pollution on one planet is probably benefitting almost exclusively the residents of that planet.
Moral hazard is an interesting concern but I don't think that it's going to end up being a severe problem.
I'm taking the attitudes you suppose might exist in the future and translating into your current context, with the intent of showing that once you consider them concretely they don't make much sense. The United States isn't going to go "Oh, it's OK if everyone on Earth gets nuked, at least our Mars colonies will still be there." anymore than you're going to be unconcerned about whereever you live getting nuked because some other chunk of your country will probably survive.
