We already "punish" people who are duped into buying stolen property (by confiscation of that property), so why should people duped by other illegal activity be treated differently?
In the case of stolen property, that's mostly because the property needs to be returned to the original owner. Zero sum and all that. You don't own the property because the thief who sold it to you did not have the rights to transfer to you; the rights are retained by the original owner.
Because this wasn't a guy fencing stuff off Craigslist, it was a multi-billion dollar global corruption that is operating in a highly regulated market. Consumer protection and all that. If someone is going to get hit in the nuts it's going to be VW and not the consumer. The cars will get fixed or VW will buy them back, but it's a ridiculous waste of resources to have police pulling over people driving VW's.
