It is because they have taken a position just as much as the theist has. Both have taken a position.
-_- not really. Like I said, most atheists are agnostic as well, so theists are saying "yes" and atheists are saying "I don't know, but there isn't enough evidence for me to conclude "yes"".
Trying to put the weight of the argument on the person that says yes is intellectually dishonest.
No, it really isn't. It's a rule that the person making the POSITIVE claim is the one that has to back it up, because the position of the negative claim is the null hypothesis. The function of a null hypothesis is thus: if the evidence doesn't support the original hypothesis or there isn't any, the null hypothesis is the default conclusion to make.
If you don't know.. then the proper stance is you don't know... not "No/Atheism".
Agnosticism is a knowledge statement, and atheism is a belief statement. Thus, as an agnostic atheist, I would say "I don't know if deities exist or not, but due to the lack of evidence, I don't believe in any deities".
You've been describing a minority among the atheist community, which is the gnostic atheist. A gnostic atheist would say "I know deities don't exist, hence why I don't believe in them". It's generally considered to be a logically unsound position to have, which is why there are very few gnostic atheists. Unfortunately, a lot of people, like yourself, conflate atheism with the stance of a gnostic atheist exclusively, and behave as if all agnostic atheists are under the label "agnostic" alone... which makes no sense, because agnostic theists exist. That is, people that would say "I don't know deities exist, but I believe in them regardless".