It isn't popular because of 'some kind of siege mentality'. It is popular because there is an observed structural power imbalance, which means that 'not joining a side' does not have 'no effect', as the 'silent' are assumed (by all involved, not just some actors acting in bad faith) to agree with 'the majority'.
Okay, think of it like this: you are not the only 'neutral' person out there. There are four sides. Side A, side B, side neutral, and side not-actually-involved. Side A will clearly win against side B, but clearly lose against side B plus some large (but not total) percentage of side neutral. Side not-actually-involved can have some influence on the outcome, but generally not much.
That is the kind of situation which I would claim things like the 'evil patriarchy' vs 'the not evil not patriarchy' are. So, there are several possibilities; if you are already a part of side a or side b, then you have already chosen a side. Whether you are part of side neutral or side not-actually-involved is pretty easy to determine: are you in the industry at all? If yes, then you are involved.
So, even assuming that you are involved, your unique help is in fact not necessary to the victory of the not evil side. Merely the help of many of the people in your reference class. In which case, if you choose not to help side b, you have indicated that you prefer side a winning over helping side b; this is not quite the same as preferring that side a win, but it is enough that 'neutrality' is usually taken by everyone involved (including other neutrals, except for you for some reason) as 'weak support for the structurally powerful/status quo/clear majority position'.
Does that clear things up at all?