I went to go look up the Grand Canyon warning signs for this post and:
The original 1990 PSAR (preventative search & rescue) sign "STOP heat kills" warning people that "under no circumstances should you attempt to hide from the rim to the [Colorado] river and back in one day!!"
It's since been updated with several versions now. (I'm excluding the one illustrating a guy vomiting. Gross.) They still mention death, but now the more common phrase is "bottom/down is OPTIONAL, getting back up/to the top is MANDATORY."
They REALLY softened this to "hiking to the river and back in one day is not recommended by the Grand canyon national park." Where before it was: "under no circumstances..."
INTERESTINGLY, I learned the following about why US national parks are soooo concerned with preventative measures which I sort of assumed but didn't know for sure:
Is a park obligated to provide search and rescue? In a 1992 landmark decision, the 10th Circuit Court of Appeals ruled in Johnson v. US Department of the Interior that search and rescue is a “discretionary function” of government that is protected under general rules of exception of the Federal Tort Claims Act at 28 U.S.C. § 2680(a). Mr. Johnson, described as an inexperienced mountain climber, fell to his death on Buck Mountain in Grand Teton National Park. His family subsequently sued the Department of Interior claiming the park failed to properly warn him of the dangers of climbing and failed to initiate a rescue attempt within reasonable time.
The 10th Circuit legally affirmed NPS testimony that “(1) the inherent dangers of mountain climbing are patently obvious; (2) both manpower and economic resources should be conserved … during emergency situations; (3) it would be impractical if not impossible to test competency, monitor equipment use, or ‘clear’ the mountain given limited available manpower and economic resources; and (4) many park visitors value backcountry climbing as one of the few experiences free from government regulation or interference.
The 10th Circuit Court of Appeals ruling further clarified a search-and-rescue team’s duty to respond, as follows:
"No statute imposes a duty to rescue, nor are there regulations or formal Park Service policies which prescribe a specific course of conduct for search and rescue efforts. Instead, the decision if, when, or how is left to the discretion of the SAR team. Therefore, the rangers must act without reliance upon fixed or readily ascertainable standards when making a search and rescue decision in the field."
All those USNPS warnings are partly because there's no duty requiring attempting search and rescue efforts if your ass ends up in trouble inside a park. If the park rangers decide they don't have the resources, or it's too dangerous, or their initial attempts don't work, they have the right to halt efforts or not begin them. Park rangers are good people. When possible, they'll definitely try to rescue you or give you an assist!
But also park rangers are there to protect parks, not people.