Personally, where someone places their license stickers has no effect on me. I don't know that I would wish $100 fine on anyone for that.
I guess that unless you're in law enforcement it probably shouldn't affect you. But, if it is a part of your job to make sure people's registration is up to date I can see where this can be a big deal.
As far as the amount, it needs to be high enough to get people's attention. I'd guess that there's a lot more warnings given than tickets though.