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
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.
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.