If the network splits, miners have a right to attack the minority fork due to transaction replay between the two changes. A bilateral split to allow two independent chains to exists requires preventing transaction replay between the two chains. No one can tell the existing protocol to fork off.
Transaction replays should be resolved one way or another. That's not an argument that justifies attacking.
A split chain with changed protocol could introduce code to stop transaction replays between the chains, but no one can demand the existing protocol to change as it is an existing entity in itself. In this case, the coin that has split away has become an altcoin. It is likely then that the attacks would stop, as there is no economic incentive to continue the attack. Attacks on a split chain without transaction relay protection is justified as it is protecting the protocol by ensuring one valid chain exists.