[danielweber]

The same reason many states say that if the grocery store accidentally overcharges you by N cents for an item, you get 10 * N cents (up to a limit) off of the marked price.

This is encouraging the consumer to engage in self-help and gives them (in theory) just enough of them the motivation to keep on the lookout for the scam.

It's not the only way to run a regulatory regime, and it has some downsides, but it also has a measure of appeal.