Why? Seems to me that ought to be the default, that money is presumed NOT involved in criminal activity unless and until the state can prove otherwise, in a court, with due process, access to counsel and all the rest. So, yeah, they should take them at their word, and if they can prove otherwise, OK. But the presumption is reversed here - presumed GUILTY, unless the owner can prove his or her innocence.
I've read of cases where civil asset forfeiture works. One example is finding someone in possession of a stolen work of art. The government doesn't have to prove the person committed a crime to seize the artwork, which seems on balance a just result to me. But that's rare, obviously, and it seems to me the abuse and screwed up incentives TO abuse the system warrant an end or VAST restriction to the process.
A while back my uncle, who is dead now but a truly messed up individual in life, stayed with my parents for a few months. They took him off the streets, loaned him a car, etc. If he was caught dealing from their house, or using the car loaned to him, the police could seize either and it would have been my parents' obligation to prove they weren't part of his drug dealing. Sorry, but that is ****ed up IMO.