It's basically a security issue.
At the auto island, you can pay at the pump, but you will probably be limited to $75-$100 per swipe, and a limit of 2 swipes. That's because they can't ID you or enter the code on the back of your card. That dollar amount limit is set by the station, but it usually coincides with how much grace the card issuer extends to the merchant. And those dollar amounts satisfy most auto fillups.
At the truck island, just about every purchase is going to exceed that card limit, and they want to cover themselves by actually having the card, the code on the back, and possibly verifying your second ID. Or having you enter your PIN.
I just stick to the auto islands, because I don't like going inside while sitting at the pump, and I can live with 2 swipes. And it's easy to do with our smaller rig.