Such a stupid, stupid rule. What gets me about it is that all balls are supposed to be called. So why make a big deal only about the 8-ball? Why not mark every pocket you shoot? Because it would be stupid, just like marking the 8-ball's pocket is stupid.
In APA, you don't have to call your shots, its any pocket anywhere... (Except the 8)
And of course, the reason for the rule is to eliminate any ambiguity about which pocket was actually called. If you've been around enough, you know that some people will nod in a direction, others will make their call in a manner that isn't specifically clear "across", etc. "That's what I meant", I can hear that now.
It supposed to eliminate issues, yet it creates others. Simpler, really, to just mark the damn thing and not have to worry about it.