Maybe because if they frequently forgive mistakes of users, they'll face mistakes from them regularly and it will turn into a mess for them. To my mind, in such a case, it's better to block users from depositing in this specific network than not to refund them when you have access to that network.
How do you block a user from depositing in a certain network if they are going to copy an Ethereum address and the deposit USDT on Optimism of which Optimism network may not yet be supported but the ETH address is compatible with that of Optimism.