Why the drop across NMOS enhancement mode load is V_t when driver is off?
It is important to understand that Q2 is in Saturation mode and not cut off mode, therefore some current flows through it (channel length modulation) which cannot be neglected. Because of the current flowing through the NMOS there is voltage drop even when Q1 is off. For Quantitative analysis we cannot ignore the current from NMOS in the saturation region.