Transmission line and termination and input impedance
The input impedance of a transmission line will be its characteristic impedance if the end terminator equals Zo. So, if Zo = RL then the input impedance to the line will be Zo irrespective of length.

If RL does not equal Zo then you get problems with line mismatches and reflections and these vary with operating frequency to cause a significant headache for digital transmission systems.

So, the assumption is that the terminating resistor RL equals the characteristic impedance Zo. This means that if RS = Zo then you have a simple potential divider.