That's just the way log probabilities work. Log(0) is negative infinity, but log(1) is just 0.

You can have infinitely high scores if you use log odds though: http://lesswrong.com/lw/mp/0_and_1_are_not_probabilities/

But that can only happen if you get every question 100% right. Whereas getting a single question 100% wrong is easy to do.