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.
Like rollbit doing asking for 200$ for wrong network depo. Happened to me on a 170$ on arbitrum network but it didn't worth to pay 200$ to get 170$ so I just give up. So there's various solutions such:
- rollbit one
or
- if kyc is done on that account to have at least 1 time the possibility to a refund
or
- minimum deposit amount
so there's quite a lot of solution atleast if they don't want to keep for them that funds

.