I've used them in Vegas, Atlantic City and Mohegan Sun and no casino has ever complained. I recall asking a dealer or pit boss about it years ago (don't remember in which town) and was explicitly told casinos don't care, as long as you hold it or wear it laminated around your neck and it doesn't sit on the blackjack table.
Is it possible something has changed in the last 5 or so years (it's been that long since I played blackjack)? I imagine it is. But I used them for more than a decade prior here in the U.S. without problems, at multiples casinos in AC and Vegas.