Why do hard forks need special rules to allow replay protection?
The LegacyCoin network and NewCoin network are not distinctly separate from each other. Unless network magic and ports are changed, LegacyCoin nodes will be able to connect to NewCoin nodes and likely many will be connected to each other. So if you broadcast transaction A-&gt;B, on the LegacyCoin network, it will likely reach a NewCoin node which then causes the transaction to also be relayed on the NewCoin network.

Furthermore, even if the networks are distinctly separate, people can operate a node on both the LegacyCoin network and NewCoin network. Without replay protection, someone can take transactions that they are hearing on the LegacyCoin network and broadcast them through his NewCoin node and vice versa.

Lastly, two way replay protection should be mandatory because the process of splitting your coins with UTXO tainting is rather difficult. You need to have decent technical knowledge to do it and you will likely need to have modified software in order to do so. It is not something that ordinary users are likely able to do without some very specific instructions and there is a high probability that they will mess it up and possibly lose coins.