It’s an OFCOM requirement, out of NowTV’s hands, for any channel showing a post-watershed programme before the watershed.
Even if that isn’t the one you are watching now, they are worried one might come up in continued watching, so you have to use the PIN.
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.