Before the United States will vouch for the security of an international flight, the other country must allow the US to assess security at their international airports. Countries like Venezuela have refused to comply with the requirement put in place after Sept. 11, 2001. Thus the US will not vouch for the safety of flights to Venezuela.
The US issues various levels of travel warnings, ranging from a lack of guarantee to a complete regulation of travel. For example, the State Department has a list of countries that have conditions making them dangerous for visitors. The list includes the likes of Colombia, Kenya, Lebanon, Haiti, Israel, Philippines, Iraq, and Afghanistan. And if a US citizen or permanent resident wants to go to Cuba from the US or even a third-party country, he or she must qualify for and obtain a license first or face civil and criminal penalties.
Would you only visit places that get a clean bill of travel from the US government?