Another example of this might be a deadbolt on your front door; it's sure not going to stop anyone hell bent on robbing you, but it makes it inconvenient enough that any 'opportunist' thieves won't bother.

At any given time, we have many conflicting desires and motivations, that are (generally) closely balanced. Desire to fit in socially and act moral (by not being a thief) vs the desire to maximise your own circumstances (by stealing someones stuff). Desire to maximise circumstances (by filling out the rebate form) vs desire to conserve energy (by being too lazy to do it). Perhaps trivial inconsistencies tip the balance of these motivations enough so that in most people, one will be favored over the other. Adding a rebate form makes tips it just enough so that your natural laziness wins out over your desire for money.

Since our natural desires tend to come from the dumb part of our brain, this has the effect of causing us to make non-optimal decisions.