Voltages are relative. What does "+/- 18V" even mean? What's the difference between that and 0-36V, or 36-72V? The only difference is which point in the circuit you arbitrarily choose to call "zero". The op-amp never sees "zero", all it sees is a potential difference between V+ and V-, which as long as that difference isn't greater than 36V, it doesn't care. You could call those rails 10000V and 10036V for all the op-amp cares, it doesn't matter.