Have the clamshell mode requirements been relaxed?

Question

I'm experiencing what I consider to be some odd behaviour regarding an external display hooked up to my MacBook:
With the external display being the only peripheral connected, the MacBook will enter clamshell mode when I close the lid.
It was my understanding (supported by documentation) that clamshell mode required output and input (a display and a keyboard and mouse; be they wired or bluetooth).
How can it make sense for a laptop to enter clamshell mode with only an external display connected?
When I close the lid with the external display connected:
the external display becomes the main display
all the windows that were open on the lid display pop across to the external
my desktop spread, littered with icons, appears on the external
the MacBook does not enter sleep
I don't have a bluetooth keyboard or mouse connected; I don't even have bluetooth on.
To the best of my knowledge I'm unaware of any settings that would affect this behaviour.
Does anyone else experience this same behaviour?
Have the clamshell mode entry requirements been relaxed in Mavericks (admittedly, I haven't used clamshell mode in previous versions)?
Is this a feature or a bug?

Answer 1

Reading comprehension is not your strong suit. Those notes are not under the heading "Opening the internal display while in closed clamshell mode." 
Here, I've copied more for you. Opening the internal display is discussed much later in the article......MUCH later.
Requirements for closed display mode:
Power adapter
External keyboard, mouse, or trackpad
External display
Use these steps to enable closed clamshell (display closed) mode:
If you are using a wired keyboard and mouse:
Make sure the computer is plugged in to an outlet using the AC power adapter.
Connect a USB keyboard and mouse to your computer.
With the computer turned on connect the Mac notebook (using the appropriate Apple adapter if necessary) to the appropriate port on the external display or projector and turn the display or projector on.
After your computer's Desktop appears on the external display, close the computer's lid.
When you close the lid:
In OS X Lion and later, the external display will change to a blue screen, then will show your desktop.
In Mac OS X v10.6.8 and earlier, wake the computer up by clicking the mouse button or by pressing a key on your external keyboard.
You should now be able to use your Mac notebook as you normally would, with a USB keyboard and mouse.
If you are using a Bluetooth keyboard and mouse on a Bluetooth enabled Mac notebook:
Make sure the computer is plugged in to an outlet using the AC power adapter.
Verify that Bluetooth is turned on by using the Bluetooth pane of System Preferences or the Bluetooth menu icon.
Pair your Bluetooth keyboard or mouse with the computer. For step-by-step instructions, see this article for keyboard pairing or see this article for mouse pairing.
In System Preferences, be sure that the Allow Bluetooth devices to wake this computer option is selected.
With the computer turned on connect the Mac notebook (using the appropriate Apple adapter if necessary) to the appropriate port on the external display or projector and turn the display or projector on.
After your computer's Desktop appears on the external display, close the computer's lid.
When you close the lid:
In OS X Lion and later, the external display will change to a blue screen, then will show your desktop.
In Mac OS X v10.6.8 and earlier, wake the computer up by clicking the mouse button or by pressing a key on your external keyboard.