Its a team game. So you can see anything that is visible to your allies, and you can (and should be able to) pan your viewport around to see that stuff. So the client needs to know everything that is visible to your allies.

Plus there are things that happen outside your viewport that affect whats in your viewport, and trying to limit your client from knowing those things would cause all sorts of graphical artifacts.

Eg. there are some spells that affect an entire area (and have graphical affects associated with the whole area). Even if part of the spell area is in your Fog of War, your client needs to know about it completely, for the graphic drawing algorithms to run. The client draws the whole thing, and then obscures the part that should not be visible to you. Otherwise, the graphic algorithms would become get additional complexity that will possibly have performance impacts.