From my desk I overheard a phone conversation at another desk where an airline flight locator asked for specification in the following way:
Coworker: We're looking for a flight on [such and such date].
Locator: And where are they flying out of?
Coworker: No, Portland, Oregon.
I mean, come now, Portland, Oregon has almost ten times the population and notoriety that Portland, Maine has. If you're going to ask for specification, at least have the courtesy to give precedence to the city that is clearly larger and more traveled.
The same can be said for Washington state and Washington, DC. At least in the circles that I've frequented, I have never met someone from Washington, DC that has referred to it as simply "Washington". I have only ever heard it referred to as "Washington" by people in films or shows. All people that I have actually met, across the states, who have been from there, have referred to it as DC. I will leave some small space for the possibility that some people very close to DC might refer to it as Washington, but I have yet to meet those people. And yet, in the hundreds of times that I told people I was from Washington, a good 90% of those people followed that response up with, "State or DC?" Good grief! If I'd meant DC, I'd have said DC. Washington state definitely should take automatic precedence, save perhaps for individuals who live within a two hour radius of Washington DC.
While we're sort of that subject, let me also inform the lot of you that Nevada and Colorado end with the "ah" as in apple. not the "ahh" sound that you make when you see something cute. And Oregon is not pronounced with a likeness of the word "gone" at the end. Think more "organ" with a hint of another vowel in the middle.
Honestly, studying linguistics greatly increased my flexibility with words and languages, no longer seeing many pronunciations as correct or incorrect. That being said, the same is not true with names. Names should be pronounced (within a single language) as close to the average of how the inhabitants of that location say it.
Update: Given the chatter on the radio/web this morning about the government shutdown, I'll amend that a possible reason for general people referring to Washington DC as Washington may result from the media referring to it as such.