A tax on poor maths skills.
EDIT: To expand, there is a cognitive dissonance here: although this is still fair, you still have a chance at winning, we feel instinctively that playing the game is futile.
This raises deep questions of philosophy. What if this same thinking is applied to everyday life? If you believe that we live in a deterministic universe, then we are all just futile scratchcard junkies, forever diligently scratching the silver stuff off the face of the universe, innocent of our predetermined (and probably crappy) destinies... If you believe in determinism and don't want to kill yourself, you have nothing to say to scratchcard/online gamblers :)
And don't get me started on Deal or No Deal.