Because they actually DID think of that. I mean, that has to be the dumbest thing I have heard posted here in months.
If you stop to actually think about the difference between a World Boss and an Instanced Raid boss for more then 10 seconds, you would almost instantly realise that Release Locking people on a World Boss would pretty much make the fight completely impossible in a PvP environment.
All it would take is for a bunch of opposing players to light your tanks up like a christmas tree (or repeat drive by nature's swiftness cyclone them and watch a dps or two die) and your entire raid wipes 1 minute later. With absolutely NO way to prevent that from happening as long as there are enough enemy players around to pick your raid off.
And the Sha of Anger is pretty much designed to be a PvP free-for all.