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.
My point is people will still make mistakes whether there are blocks or even warnings.
He will end up losing the funds. A similar thing happened to me, I depo 170$ on rollbit but on arbitrum network by mistake (in-case you want them refunded you've to pay rollbit 200$ so sometimes it not worth), so sometimes we've just to give up.