Last week I swiped my card at a gas station pump before noticing the tamper proof seals had been broken.
Most likely from some brat kid breaking them with his fingernail.
The real reason for the seals is to stop people from getting free (or almost free) gas. If you know how to operate the pump side controls, you can press a couple of buttons to change the price to pennies per gallon. They're only downloaded when the price changes, and the back-end system then only selects which price level (cash or credit) to use for a transaction.
And if you're using debit, the PIN pads are supposed to be potted and keys injected into battery-backed RAM (code in RAM is a good idea too) to prevent tampering. In the US at least, the key is supposed to be only known by the clearinghouse at the other end of the comm line, and only comes out of the pad already encrypted.