It's that the receiver functions as an HDMI repeater. Since it isn't a display, it has to pass on the resolution information of the display connected to it. So when the display turns off, it becomes an end device, and re-handshakes with the source with the updated information.
I've seen devices that go between the receiver and display that are always on, and lock to the display's information. These are mostly to keep the receiver advertising at least 1080i so you can get multi-channel audio at over 48 kHz (8 channels of 24/94 audio is too much to multiplex with 480p video, which most receivers default to). I just usually tell my Pioneer plasma to turn off the display, while keeping the rest active so I don't get the audio drop-out, and can maintain full quality.
