That's because you use a different definition of the word as to how people use it.
When debating an issue, there tend to be 3 significant positions.
There's agreeing, disagreeing and not quite sure either way.
In the first two you have an opinion either way and the third shows a lack of commitment to either - a suspension of judgement.

People tend to use 'theism' as agreement with "God exists", 'atheism' as disagreement, and 'agnostic' with no clear opinion either way.
When you tell people they have to be either 'theist' or 'atheist' you are basically being a language Nazi as to how they should and shouldn't use words.
(To be honest, I don't think you can justify that they 'ought' to use the language your way, but if you have some justifications then give them a go)

It would probably be more productive to point out what you and the 'atheist community' tend to mean by atheist.

As to the 'us' and 'them' issue, it's a natural psychological accompaniment to any position you hold strongly.
8 years ago I used to think in terms of 'Us' as Nintendo fans and 'Them' as Sony fans.
(That still lingers a little, and probably would even if I myself bought a Playstation! )
Personally, I think that the 'us' and 'them' state of mind isn't good - it'll cause you to project prejudices.
Having said that, if you're fighting for a cause that needs to be fought, I guess you have to accept that such side effects will occur.
Being aware of them should limit the damage...