Ode to credit card payments using machine or human

I travel a lot. And as a result, I fly out of Houston Hobby airport and become very efficient and getting there, parking, and making a fast get-a-way. Something occur to me a few weeks ago as I was peeling out from the parking garage to head home.

When paying for parking, you have a couple of options. You can use an auto teller and swipe your credit card through a machine, or you can do it the old fashion way and interact with a parking attendant who will gladly process your credit card.

The issue is, when I use the machine, I insert my parking ticket followed by the credit card and usually 15 seconds later, a receipt is printed and I retrieve my credit card. Now, in the event that the automatic machine is down (which is often), I have to go to the parking attendant. I give them the parking ticket, a total is calculated, I give him my card, he swipes the card, I wait about 1 minute (I've timed it), he hands me clipboard, sign the receipt, and then get my card back and my copy of the receipt and then I'm out of there.

My question, why is it that I have to sign a receipt when using the parking attendant but do not have to when I use the automatic machine? I've heard that transactions under $25 don't require signatures, but most of the time, my parking ticket is on the order of $50. What gives?