"This fee of $10 will be applied if you return your car to a different location from that which was scheduled, or if you return more than 12 hours after the date and time previously scheduled, and you notify us of an extension of your rental by the return date and time previously scheduled by calling 1-800-654-4174.
If you do not notify us of such a change, the Late Return Fee of $12 per day, (maximum of 5 days) will apply."
Which is what will happen here. They'll charge you this fee, but also adjust the base rental charges to much higher rates that reflect the fleet management burden caused by one-way rentals.
In practice, the cost usually turns into around $0.37/mile, plus the daily rate, unless you have a corporate discount.
My (very limited and probably outdated) understanding of the rental car industry is that the locations are franchises and the cars are owned by the franchise. If you rent a car round trip and leave it in some other city they have to deal with getting it back.
(Though I do wonder if "Hi, I'm a photographer and I lent my camera on the internet" and "Hi, I'm with the loss department of a major national rental car company" get you different responses from the police for reasons entirely unrelated to the legal status of the misdeed.)
You shouldn't project your obliviousness on everyone else.
Vancouver, WA is a suburb of and part of the greater Portland area. Also referred to as the Portland / Vancouver metropolitan area.
It's completely understandable that the "flunkie" at the counter assumed someone leaving from the largest city in Washington would be be referring to the 4th largest city in Washington (Vancouver).
Sounds like the big fee was well deserved.
// Except it's not, they're filmed in the Canadian Vancouver anyway.
I will concede that some people in Portland call it "Vantucky" because it's trended more conservative.
Better to ask forgiveness than permission.