Apart from the definition of "atheism" being the absence of "theism", linguistically (theism being a belief in the existence of God; thus, "atheism" must be the absence of that belief), I think that the theist community often tries to put words into the mouths of atheists by claiming that they actively believe that God does not exist. This, in essence, erroneously turns "atheism" into a belief system. For example, there is a pretty well known debate podcast called "Dogma Debate". The host of the show is a very outspoken atheist, BUT he has said time and time again that he does not actively believe that God does not exist. He simply explains that he has not been presented with evidence pointing to this conclusion. Thus, he is a "weak" atheist. I would say that most atheists, whether they have explicitly expressed it or not, share this view. Because of this, I feel like the broad definition is necessary to avoid confusion. Atheism is not a world-view, nor is it a belief system. It is merely the lack of one single belief. This can be easily shown by looking at some of the "atheistic" religions (or "world views") that currently exist in the world today. They are "atheists", and they are part of an organized religion with certain beliefs, some being supernatural in nature. If we were to change the meaning of the word to describe a belief system, it would undermine many of those who identify as "atheist".
The majority (theists) should not get to define the position of the minority (atheism), which seems to obviously be the case. They want to hold atheists to something they are not so they can look down on them as illogical, when, in actuality, the lack of belief is completely reasonable. On the other hand, the case for actively believing that God does not exist is much more difficult. I see this as an attempt to hinder atheism as an identifying term, making it seem more unreasonable than it actually is.
Those are my reasons. And, fyi, I must have missed where any valid argument was given for changing the meaning of the term "atheism", making it a belief system rather than a simple lacking of a specific belief. The reasoning I saw was flawed in that there was an attempt to equate a lack of belief with a negative belief, which is a logical fallacy.