Why must the event horizon be a null surface?

[center o bass]

The event horizon is a null surface in spacetime separating events that are causally connected to an observer from those that are not. Does anyone have a good, simple argument on why it must be a null surface?

 

[PeterDonis]

You gave the key fact in your OP: the event horizon is a causal boundary. Any causal boundary must be a null surface, because causal influences can travel at any speed up to the speed of light, but can't travel faster than light.