Yeah, that stood out for me too.  OK, I'm white, but I have worked with Indians (with the Forest Service) for a long time, and have heard both Indians and whites (on rare occasion) use "rez" as short-form for reservation, and it never raised an eyebrow.  It's just inoffensive, casual slang, as far as I know.  I've never said "rez" myself because it would sound so presumptuous, even though I am on reservations now and then.  I mean, who am I kidding?  The rez?  But I doubt that it's a racist term if you are a non-Indian.