Because the Figure 1 you posted refers to a 4-terminal device, not a 3-terminal one. If you look at the schematic symbol in Figure 1, you'll note that the body terminal is a separate terminal not connected to the source terminal. MOSFETs for sale are almost always 3-terminal devices where source and body are connected together.
If memory serves me right (not 100% sure -- seems to be corroborated by this handout), in a 4-terminal device there is no difference between source and drain, and it is the gate-body voltage that determines the on-state of the channel -- with the caveat that the body is supposed to be the most negative voltage in the circuit for an N-channel device, or most positive voltage in the circuit for a P-channel device.
Look carefully at the conventional schematic symbols for N- and P-channel MOSFETs (from Wikipedia):
n-channel http://upload.wikimedia.org/wikipedia/commons/thumb/6/62/IGFET_N-Ch_Enh_Labelled.svg/80px-IGFET_N-Ch_Enh_Labelled.svg.png p-channel http://upload.wikimedia.org/wikipedia/commons/thumb/0/0c/IGFET_P-Ch_Enh_Labelled.svg/80px-IGFET_P-Ch_Enh_Labelled.svg.png
and the Wikipedia figure on MOSFET functioning, and you'll see the body-source connection.
