Essentially, a limitation of zone-wide or general chat should be implemented in the cyrodiil zone. This means that you would only be able to communicate with other players (friendly or otherwise) at short ranges. Wispers/direct communication are harder to limit, unfortunately.
The goal is to create a need for messengers/runners/scouts to inform groups of players of enemy forces and movements. If zone-wide communication was available, everyone in cyrodiil would immediately know of any attack on a hold, thereby detracting from the feeling of warfare between the factions.
Instant communication doesn't exist here, outside of magical means, and keeping chat in check this way would support ambushes or surprise seiges against enemy facton holds, without them being able to instantly call their entire faction to assist.