I don't think they care... so long as they have the code to put in the machine!
At one cinema near me, when it's really busy, I have seen people walking along the queue with clipboards writing down the Orange Wednesday code and handing out a BOGOF voucher for use when you get to the till.
Presumably a time-saving measure (I'm guessing somebody sits and enters the codes later when it's quiet), but it would be easy to edit the date and number on the text and send it to yourself our a friend! I guess they go on the basis, that 99% of the codes given turn out to be genuine and they don't lose-out much.