The legal theory (which is a total crock and obvious constitutional violation) is that you aren't the real owner, you stole the property so the state can take it away from you. That's what creates the presumption of guilt where you have to prove you're the owner of the property, same as if you dropped it on the street by accident and the cops found it.
Anytime one of these cases gets close enough to SCOTUS for a decision on the overall constitutionality the police/DA drop the case. Most of them seem to know its a scam and don't want to risk an adverse ruling. There have been a few hints that SCOTUS is ready to rule the current scope and scale makes civil forfeiture unconstitutional but who knows.