According to the schematic, buttons S1 and S2 are just connections between P1.2/P2.6 and ground. This means that the GPIO pins need pull-up resistors to have a defined state when the button is not pressed. In other words, the PxREN and PxOUT bits must be set. I don't know why it works without the pull-up for S1, but it's probably just luck. Don't rely on it.
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