You already have to show ID to see rated-R movies in the US. I don't see how this is any different.
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The law doesn't, in most places, require theaters to demand or log ID (it sometimes requires them to deny admission to people under 18 without parent or guardian permission, and in some places doesn't even do that, with any restrictive policy being a matter of theater policy following private industry group recommendations), and they mostly don't even do the former unless the patron appears, to the ticket seller, to be underage (and even then, IME, its iffy, probably because while that's generally theater policy, the ticket sellers aren't minimum wage earners, likely teens themselves, and not closely supervised.)