From my interactions with atheists, they often prefer the "having no belief in god" definition for a few reasons:
(1) on this definition, even babies are technically atheists, making it sound like it's the default position of human beings,
(2) it sounds like they're not being too rigid in deciding either way - they just don't happen to have that belief, like I don't happen to have blonde hair, and
(3) it means that they haven't had to do the hard work of grappling with the arguments for God's existence - they just 'lack belief' in God, in much the same way that I 'lack belief' in dragons.
The reality is, atheism - however they wish to define it - is an active position of the rejection of God.
The Apostle Paul got it right: