Probably because it's perplexing. It's analogous to someone who is disinterested in sports registering for a web forum that discusses sports. It quite understandably begs "Why?"
Atheists are commonly well-versed in religion. Not sure where you'd get the idea they're neccessarily disinterested.
Besides, religion is a powerful force in the world regardless of the truth it holds. That speaks both to its nature and OUR nature as humans.
Though honestly, I don't know anybody who isn't religious, so I would not personally ask this particular question. Everybody has some sort of system from which they derive personal meaning and their way of life from. Whether or not they actually call it their religion is unimportant to me; the function is equivalent.
That is something I've both heard and been perplexed by before. We all have our own morals, but denoting this as religious appears strange to me.
I'd be at a loss to describe my own 'religion' in this sense, and the lack of that ability suggests to me that I'm not religious.
But then, I get confused by certain definitions of God as well. I don't worry about it too much, just find it...well...confusing. I don't get why people stretch definitions beyond general acceptance when there are other ways to describe the same thing.