There is a massive difference between arresting someone holding a load of cash in a bag running down the street from a robbery and trying to impound the assets of people who have not been arrested or charged with anything on the off-chance that an investigation, still in its infancy, might say they had a case to answer especially when the assets themselves are part of a pot of monies held by those individuals/organisations.
As I recall, this was a magna carta thing. The King couldn't take all your property and then think of some charges afterwards.