The case example you place has no chances of happening really. If a particular account does something dodgy, and authorities require the Exchange to seize his funds, they seize them at the persons account logical level on the Exchange.
In the hypothetical scenario stated above, user has already withdrawn his fund from exchange, after converting it to Bitcoin.
U then converts S to Bitcoin B and withdraws from E.
So, freezing user's empty account is of no use. Also, as per the news links given in OP, authority did not care who own the address. They just froze it first out of the blue. So, I wonder, why the case sample I placed has no chances of happening reality!