I don't see any philosophical reason to conflate the conventional distinction between atheism and agnosticism, although I think that there exist some political and social pressures to do that in contemporary American society: some individuals who affirm atheism in private are more comfortable inaccurately describing their attitude in public as agnosticism and some theists critics wrongly label both categories of failure to believe in God as atheism. There probably are interesting historical and sociological stories to tell about those pressures, but I don't think there is a compelling philosophical story about why redefining atheism in the manner you describe is rationally required or that suggest that this would be a useful revision to make to our language.

Mark's reply raises some interesting issues about human rationality. If atheists were required to have considered multiple arguments and assessed lots of evidence, then it would seem odd to describe many human beings as atheists and not just squirrels -- and that conclusion itself seems rather odd (and, indeed, mistaken) since human beings can (and frequently do!) form the belief that God does not exist without having undertaken those sorts of intellectual activities. Consider, for exampe, a sub-culture where it becomes trendy to embrace atheism, in which case one could imagine individuals doing so out of a desire to conform or to gain social acceptance.

I think the relevant difference between stones and squirrels and human beings is that humans can have beliefs about God's existence and the others apparently cannot -- in the one case because stones can't have beliefs at all and in the other because squirrels seemingly don't have rich enough mental lives to have beliefs of that sort. To be sure, human beings' rationality is arguably essential for our having mental lives that can support beliefs about God, but this doesn't mean that humans can only gain those beliefs by exercising sophisticated reasoning. Even if our rationality is essential for our mental lives being as rich as they are, it could be the case that some beliefs that play important roles in our mental lives aren't based on reasons or evidence at all, and it could be the case that some individuals' beliefs about the existence or non-existence of God are like that.