Probably because dudes that had their cars impounded would rather do what they could to pay up and get their cars back before their wives found out or if their wives already knew, just pay up and keep it quiet before it was broadcast in the news that so-and-so was out trolling the streets for hookers...

Seriously though I seem to remember a case where a wife sued a few years back because her car was impounded due to the husband's nefarious activities. I think the case was thrown out because of the law.

The other big "victim" of this law would be (ahem) "unlicensed pharmaceutical distributors" who need to keep large amounts of cash on hand and don't necessarily have purchase orders or inventory receipts. Although the Feds are more aggressive on this the state has also got into the act, seizing cash and keeping it, regardless of whether the individual is actually found guilty of being an "unlicensed pharmaceutical distributor"..