The Iraqi people had a dictator who slaughtered his own people, etc. If the "foreign occupiers" leave at the wrong time, they could easily see the rise of another.
For example, the removal of the Shah led to the as repressive, if not more repressive, Ayatollahs. The people of East Germany went from Hitler to the Communists.
As discussed above, the US wants out of the country at the appropriate time. That is quite different than either a colonial policy or abandoning the country to a potential new dictator who wants to stay for life.
