I agree. But atheism isn't one - it may be a part of one, but atheism in and of itself does not constitute a belief system.
You've not really addressed my point. Saying "People have belief systems" does not mean "X is a belief system".
you have a negative opinion of the word belief, atheism is a philosophy, a belief system that the individual identifies from. saying i'm atheist, or i'd don't believe in god(s) is not saying i don't have beliefs. beliefs can be negative/positive.
an atheist identifies from the premise that there is no god, and then constructs his/her world view, a theist identifies from the premise that there is a god, or something divine, and constructs his/her world view.
one theists belief system is not going match another theists world view, or belief system, 100% anymore than one atheists philosophy is going to match another atheists.
belief systems are subjective and are how the identifier observes and interacts with her/his world.
http://www.thesaurus.com/browse/philosophy?s=t