They had $88,000 in cash. At the time it was seized it was unknown and irrelevant whether they were "drug dealers" or if this was "drug money". It was seized because it was "too much cash".
It's not too hard to figure out that the bureaucrats have an interest in making people conduct as much of their financial affairs as possible through banks because it makes the details of those affairs easily available to them. The don't want people conducting any substantial amount of their financial affairs in cash, and will do everything they can to discourage it.
Doesn't have to be "substantial" either... look at the EZPass-only exits now being constructed on some Interstate highways, so the note on our currency "[T]his note is legal tender for all debts, public and private" is meaningless...you must get a device that allows the government to track your movements, if you wish to use certain public roadways.