> It's like when atheists are dumb enough to treat all Christians alike, or Muslims, ...
No it's not like that at all.
When you join an organization that espouses certain values, then you must agree with those values. Otherwise why would you join?
For example, if somebody joins the KKK, it would hardly be wrong to think that person is a racist. And if somebody joined NAMBLA, then it is fair to believe that person believes it is okay to molest children.
Atheists have no set ideology. For that matter, theists may not either - unless they belong to some organization that has some specified sort of ideology.
But if you are Christian, Muslim, whatever; then you are claiming that you ascribe to those values.