For Kirchoff's Law, a node is any bounded region, not just a point where two or more conductors are connected. In this case the entire 3-terminal regulator with input, output, and ground is considered a node. The Law quite simply states that all the current entering a bounded region (a node) must be equal to all the current leaving the bounded region. It doesn't matter what kind of circuitry is inside the region!

Why is this so? Because otherwise there would be an accumulation of charge, either positive or negative, from the excess of current entering or the excess of current leaving the region. You think a Van de Graaf generator is a counter-example to Kirchoff's Law? Think again. All those charges (current) being carried on the belt to the top of the generator eventually come back down when current is drawn from the charged sphere at the top.

Kirchoff's Law is used together with Ohm's Law to analyze circuits. No matter how many parallel and series connected components you have, application of these two laws will allow you determine the voltage drop across and current through each component.