Returning the money doesn’t happen all the time. In fact, it could put the exchange at greater risk rather than freeing them from legal implications. The implication here may be that the exchange is aiding criminals by sending their BTC back to them.
The best an exchange can do in such situations is to:
Freeze the account temporarily
Demand KYC from the owner
If found guilty, they report the whole situation to the appropriate authorities (this is much safer than returning the BTC)
The funds are then seized by the right authorities
Is there anything a defaulter can do at this point?
No, there is not much he can do. The funds are lost, and he would have to be extremely careful next time to avoid violating the rules and regulations of the exchange and the government.
When you already deposited your cryptocurency to a centralized online platform (exchange, gambling site, whatever), you must afford to lose it. Because they can be a scam platform or a good one, but if they close your account, seize your coin, you have nothing to do.
Reminder: do not keep your money in online accounts.If you don't commit any crime, you can try with filing a court case but most people are not able to do it financially so they will just give up and lose money.
Exchanges have many reasons for deaths like scam exits
https://www.cryptowisser.com/exchange-graveyard/