I'm a server at a chain steakhouse. Let me chime in. With the computer system that we use, whenever the automatic gratuity is put on (18%, parties of 6 or more), it shows the gratuity on the itemized receipt. When you run a credit card, it will print up in the area where the tip line normally is something to the effect of "A gratuity has already been added for your convenience." Unfortunately, if, during the process of authorizing the credit card, you either exit out of the table, or reprint the receipt, the credit card slip will print out without the notification of the gratuity.
I know when I'm busy, I've noticed that I've exited out of the screen while it was authorizing and therefore it didn't print out the gratuity information. That said, if I notice this, I inform the guest about the gratuity and thereby use the tip line as an additional tip line. So, it CAN be the result of an honest mistake if the server wasn't paying attention.
That said, there's obviously some dishonest servers out there. And I'm sure some are doing it on purpose.
As a side note though, everyone please remember the auto-grat is not the ONLY tip that can be given. If you get the best service of your life, and see an auto-grat is on the ticket, don't just sign the ticket and assume 18% will thrill the server (I know, I know, we're happy with ANY tip, right?
). Even if there's not an additional tip line, you can always add (and take away for that matter, in cases of particularly bad service).
Chris