There is one big difference between a former atheist who converted to a religion (especially an evangelical one) and a former believer who figured his way to atheism.
The now-atheist doesn't believe that there is some invisible figure giving him 'points' for trying to convert people over. Yes, there are the truly vocal ones, but you can ignore them if you choose, if they are authors. If they are Michael Newdows trying to make asses of themselves in the courts, well, they would have found some other way to do it anyway.
For whatever reason, many atheists try to de-convert Christians anyway. I see them on Christian message boards all the time. It puzzles me though, since I never bothered searching out Christians when I was an atheist (and yes, I strongly believed that there was no god).