Contrary to common law, eight states have laws requiring people to help strangers in peril: Florida, Massachusetts, Minnesota, Ohio,[statute verification needed] Rhode Island, Vermont, Washington, and Wisconsin.[statute verification needed] These laws are also referred to as Good Samaritan laws, despite their difference from laws of the same name that protect individuals that try to help another person. These laws are rarely applied, and are generally ignored by citizens and lawmakers.
And you're right, other than those states there is no law requiring you to stop and render aid (unless you directly caused the incident. thats called "rescue doctrine"). You should still stop and give them a drink and call the cops on them out of basic human decency IMO. Doesn't mean you should set up water stations and hand out maps, nor should you traipse about the desert looking for a thirsty people. Just that should you encounter one, you should extend the basic human kindness of a drink of water.