They defend it by aping the way atheists usually present their case against religion; no evidence. Of course, that's really silly when they're the ones making a claim, but that's beside the point.
They are saying that having no evidence is bad (I'm using good/bad in a really generic "debate terminology" sense). That's the basis of their attack on atheism, once they've succeeded in mischaracterizing what it actually is.
Fiddling While Rome Burns
Carthago Delenda Est
"I used to roll the dice; see the fear in my enemies' eyes... listen as the crowd would sing, 'now the old king is dead, Long Live the King.'.."
"It ain't what they call you, it's what you answer to." - W. C. Fields
There should be Instant Runoff Voting
My first inclination is to wonder if you were mis-interrpreting their argument. They may have argued against a god in particular (i.e. the Christian one). That's entirely possible to do, because Christianity sets forth a bunch of specific characteristics of that particular god, and many of them can be proven or disproven -- even just logically.
It's entirely possible to disprove the existence of a particular god, at least in the specific way it's presented. It just isn't possible to disprove the entire concept of some kind of god existing.
No. Since there is no one ideology or set of behaviors to which all atheists adhere it cannot be called a religion. Not all beliefs are relgious beliefs. I believe I'll have another beer.
“The reasonable man adapts himself to the world: the unreasonable one persists to adapt the world to himself.
Therefore all progress depends on the unreasonable man.” ― George Bernard Shaw, Man and Superman