I think it's because they're more likely to have read into both sides of the argument, as most Christians I know couldn't tell you the first thing about evolution whereas most atheists could tell you all about it and even quote the bible in places.I think johnstoni and zebedee101 have summed this one up quite nicely - why is it always the atheists that speak the most sense in religious debates!
And Johnstoni, I think the yellow lab tied up is a breach in Biaza? Also I'd guess the hybridising zebras on purpose is, and the rhino house is mighty small.