The Olympic facilities and surrounding residential areas in Rio de Janeiro are neither safe nor sanitary. There have been far too many crimes committed near the facilities and hotels for anyone to feel safe. It's well known that the area is severely contaminated with waste and filth. There is significant speculation that even the police forces in Rio can't be trusted with upholding their laws and some think they may be part of the crime problem. The country of Brazil itself is undergoing an economic collapse and political revolution.
For these reasons, any one of which should be sufficient, the United States should boycott the 2016 Summer Olympics.