Correct me if I am wrong but if the rental appears in the iTunes library then technically it is not transferred.
I have purchased but never rented movies but from what I have read, your assumption is correct. Once you move the rental to an iDevice it should disappear from iTunes.
If you download a rented movie on your computer: You can transfer it to a device such as your Apple TV, iPhone, iPad, or iPod if it’s a standard-definition film (movies in HD can only be watched on your computer, iPad, iPhone 4, iPod touch (4th generation), or Apple TV). Once you move the movie from your computer to a device, the movie will disappear from your computer's iTunes library. You can move the movie between devices as many times as you wish during the rental period, but the movie can only exist on one device at a time.
Taken from this website....