The only reason I can give for stating that I am an atheist and not an agnostic is probability!
An active disbelief in god is as you say the direct opposite of an active belief in god, there is no empirical evidence either way (well there may be but I don't know enough about it) but because two views are opposite they are not both necessarily equally likely.
There is two sides to the arguement but one of them has to be wrong!
I can only use my life experience and the evidence that I have read about and witnessed in other peoples life to make a decision as to what I believe (or don't believe).
I can't prove there is no god in the same way that it cannot be proven that there is, all I can do is live my life in a way that honours the most likely situation as I see it (as you can probably guess that is an active disbelief in any deity or creator).
Agnosticism seems to me to be a bit of a cop out but that is just me.
I also believe that the onus of proof lies with believers.
