Atheists recognize that taking a strong position--absolutely, positively, there is no god--comes across as dogmatic and intolerant.  Although many atheists espouse the strong position, the leaders of the atheism movement prefer the weak definition--there is no credible evidence showing that God exists--not only  because the strong position appears intolerant, but also because "it does sound rather untenable."  They acknowledge that the most persistent objection to the strong position of atheism is that it sounds dogmatic and unscientific.  Advancing the strong position in public debate forces all atheists (both strong-position and weak-position) to prove the nonexistence of God, invoking the burden of proof. 

Atheists are quick to acknowledge that the strong position has disadvantages in public discussions at the popular level because it is easy to portray as dogmatic, unreasonable, and thus unscientific. To avoid public relations and marketing embarrassments, the atheism movement tries to show that the strong position of atheism, far from being the only form of atheism, is the rarest among atheistic positions.  Instead, they advance the weak position of atheism.  From this perspective, they shift the burden of proof to the theists.  Here is how Positive Atheism magazine describes the ideal sequence when an atheist talks to a theist about the existence of God.

• It must be realized that we are dealing entirely with claims -- claims that various deities exist.
• In discussing such claims, it is always the person making the claim [the theist] who is responsible for providing evidence and strong argument. 
• The person listening to the claim [the atheist] need not make any argument at all. 
• The listener [the atheist] does not need to disprove a claim in order to reject it. 
• If the person making the claim [the theist] fails to make a convincing case, the listener rightly rejects the claim as falsehood (or suspends judgment, based upon the strength of the claim).   In either event, the listener ends up lacking a belief in the object of the claim.
• It is never the negative [weak-position] atheist's responsibility to prove or disprove anything. That job belongs to the person making the claim, which is the theist.

Don't get the wrong impression.  Atheists believe that the strong position is defensible; they just prefer to shift the burden of proof on the theists.  Atheists can articulate the nature and range of their non-belief, and they can discuss their reasons for rejecting theism.  They do so primarily be dismissing the religions of theists as being beliefs that consist entirely of controversial and untestable claims.