Torn bar receipts

Why does the server tear the receipt after they bring you your drink in a bar? And why does this happen most often in airport bars ?

Inspired by opus in the ‘Body cavity search’ thread.

An intact bar receipt is your voucher for a drink you have ordered/paid for but have not yet recieved. A torn receipt is your receipt for a service/product that has been fulfilled/delivered. Airport customers are often traveling on expense accounts and want receipts for everything.

Thanks Ursa,

I have never gotten a untorn receipt, which was why I was confused. I thought maybe some day I would get a virgin receipt and get to wave it frantically as my aircraft was boarding, while the server flirted with the maintenance crew that just got off.