but how are they going to charge the 20 USD from him per month if the assets in there are just $0.5?
They would do the same as his bank would do if his account was in the red: they would send him a letter to the address he provided when he registered, informing him that he still has outstanding fees according to the terms and conditions and that he needs to pay that money.
But at least for EU customers, this fear does not apply.
From their FAQ:
Quote
I only hold small balances in my account; will the default fees take my account into the negative?
For customers that hold an asset balance that is less than the minimum fee, the whole balance will be taken, and no additional amount will be charged thereafter. We will not take your account into a negative balance by charging fees.
For customers that hold an asset balance that is less than the minimum fee, the whole balance will be taken, and no additional amount will be charged thereafter. We will not take your account into a negative balance by charging fees.