In the future, I think it's gonna get much more difficult for banks to trace where exactly the BTC came from.
It sounds like you're thinking as a bank, well they're not in difficult position, they're not at risk.
They have a rule like if the customer failed to show the evidence, the banks will give penalty which could make the tax double or triple than the actual tax. The banks can also have an option to seized the coins, so they don't need to work harder to handle this case, the customer is.