My experience - in physics - is that many publishers [1] *ask* for a publication charge. That is, your article gets published whether you pay or not. Paying is voluntarily.

I don't know about CS, though.

[1] E.g. APS (Americal Physical Society) and AIP (American Institute of Physics) do it this way. Except Nature and Science, those two publish the journals a physicist wants to appear in.