Perhaps because his narrative is that she was picked up by abductors - perhaps because she will eventually be found at some other location. If just lost in the desert, but then found dead somewhere closer to say, Vegas, he'd be looked at as the prime suspect. This way, he's setting it up so that when/if she IS found, there's a reason for that. And it's nothing to do with him. Perhaps because it would make himself appear negligent somehow, considering he held the water, her skin was unprotected, her age, the heat, etc. This way, everything happened so fast, she was within sight of a cooler refuge with water, he was just packing up, and only 5 minutes behind her, but she couldn't wait for him, and someone just couldn't resist taking her. He's blameless and seeks compassion.