Art of Electronics 2nd edition p. 67, figure 2.8 -- what causes the clipping?
The load is always &quot;out of the circuit&quot; when we are figuring the amplifier output impedance.
The transistor can only control current flowing from the positive rail, the best it can do in the other direction is to stop conducting altogether. At that point the transistor is not involved in the circuit and you  have a 1K resistor to the negative rail. The output impedance of the amplifier is 1K and you can only get half the power supply rail voltage across the 1K load.