The difference is that people who commit crimes like robbery do so out of economic need. Locking people up for crimes that arise out of poverty isn't going to address crime, because the environment those people were living in will continue to exist and will cause more criminals to come into existence. The threat of punishment self-evidently does not prevent crime; after all, people commit crime in spite of the fact that it's illegal. The solution isn't to punish criminals and continue to perpetuate the tragedy that is our vengeance-fueled justice system, it's to provide these people with educational and employment opportunities.

On the other hand, someone who covers their license plate has demonstrated their depraved indifference to human life and is irredeemable as a result. You can't reform someone who does things like that. It makes sense that people would get a lot angrier over it.