Did you mean the disabled states to be "Z" instead?
Rather than strict boolean logic, circuits are typically designed with "strength" levels. The IEEE 1164?? standard has six states, if I remember right: X (don't care, can be synthesized to any of the others without concern), Z (undriven, floating, high impedance/Z), H/L (weak pull up/down) and 1/0 (hard pull up/down).
The reason for this is conflict resolution. Two outputs should never drive a common bus at the same time, at the same drive strength. Thus 1/0 at the same time is a conflict, or H/L at the same time is a conflict. But 1/0 dominates over H/L, which dominates over Z.
For example, a wired-AND circuit consisting of a passive pull-up and open-collector drivers (e.g., I2C bus) can be described as default 'H' (the passive pull-up), and active '0' (when any one or more drivers asserts the line). The drivers, in turn, are default 'Z' (open circuit), active '0'. The default-high state arises because 'H' is stronger than 'Z', and the active-low state arises because '0' is stronger than 'H'.
Tim