The Heritage Foundation has produced a helpful factsheet that explains civil asset forfeiture.
No. 4, especially, stands out:
Being innocent does not mean that a state has to return your property. The Supreme Court of the United States has held that the “innocent owner” defense is not constitutionally required. Furthermore, even in states where you do have an innocent owner defense, the burden is typically on you. Your property is presumed to be guilty until you prove that you are innocent and that your property therefore should not be forfeited. In other words, you must prove (1) that you were not involved in criminal activity and (2) that you either had no knowledge that your property was being used to facilitate the commission of a crime or that you took every reasonable step under the circumstances to terminate such use. And all the while, the police retain your property. To cap it all off, the success rate for winning back property is low. Pragmatic property owners, however innocent, may reason that it is best to cut their losses rather than challenge the forfeiture in court.