Tire inflation (PSI) range for offroading?

[H3 Hummer]

When I went off-roading last weekend, the Park Ranger told me that I would get much better traction if I took some air out of my tires. I asked him about what level and he said 15 - 18 PSI.
His tires looked about the same size as my H3 (if that matters).
That tire pressure seems real low but the idea for flatter, larger foot print makes sense. If that range is true why does the Hummer beep at me under 25 PSI? Is that warning feature only for highway driving? What should the PSI be for off-roading driving? 32 - 33 inch tire size.