Murder and emissions fraud are like apples to oranges. Emissions fraud are a corporate practice, while murder is a one-off event. Coming clear on the latter improves all industry.
For the record, there have been cases where criminals have come forward for crimes after statute of limitations expired. I am not sure it if was in particular to murder though.
I think you get my point, though. Amnesty for rule violations does not discourage future violations. "Everyone's cheating on emissions testing and there's no penalty" isn't going to dissuade anyone from cheating on emissions testing.
You can compare it to "you are allowed to murder anyone you want, as long as you confess afterwards" and see that that's not the best public policy if your goal is to reduce murders.
"The penalties for murder are too harsh, so people aren't confessing to many unsolved murders."
Well, yeah. I would not blame "the system" for this, however.