Yes US citizens do need a Valid Visa to Visit Cambodia.
Not only with the US citizen need a Visa to visit Cambodia the US citizen will also need a valid passport to visit and enter Cambodia.
You'll need to get a Cambodia Visa and a Passport before you are allowed to enter Cambodia or you'll be denied entry.
Tourists and business Cambodia Visas are valid for up to one month from the date of entry into Cambodia.
As of now Cambodia is is currently one of the poorest countries in the world.
The average annual salaries for most workers in Cambodia is $800.00 to $1,500.00 per year which is a pretty low salary.
Most people in the USA make at least $800.00 to $1,500.00 per month and sometimes more.
If I were living in Cambodia I would be very poor and not be able to survive as much.
Especially if I only earned $800.00 to $1,500.00 per year in the USA I would hardly even be able to eat and would be living on the streets.
So while the USA can seem like a terrible country to live in other countries such as Cambodia would be much worse to live in.
Visiting Cambodia is nice but I would not want to live there.