You literally just said "believe" because that's exactly what assuming the complaints are made in good faith is
Responding as if someone is acting in good faith means that
- you believe that
- they believe that
- they are true
On the other hand, if you believe someone it means that
- you believe that
- they are true
Using "believe" to mean the first situation only makes everyone's meaning less clear.
That should give you pause when you claim (incorrectly) that a such a narrow usage of language is correct.
All three points lead to different actions and the one in the middle seems to be the most appropriate.