Because this is what is happening to the Horde on Burning Blade EU. BB is being connected to Drak-Thul. BB Alliance and Drak-Thul Horde are well known Czech servers. (See guild descriptions at http://www.wowprogress.com/pve/eu/drak-thul). But the underpopulated sides are English speaking.
Blizz have paired these realms because they know they are Czech speaking. Its fine with me to have a Czech realm, but Blizz have completely ignored the English speaking factions in this connection. The English speaking BB Horde and DT Alliance are going to lose the ability to communicate with others on the server. The right thing to do would be to allow the English speaking players to free transfer.
Is it okay for Blizz to ignore us? The MVPs on the official forums certainly believe so. Paid transfers or stick to only talking in your guild.