أَثْنَيْتُ عَلَى عَمَلِهِ، لَا عَلَى كُلِّ أَقْوَالِهِ.
I praised his work, not every one of his statements.
The first assertion could be misheard as unqualified endorsement of everything the person says or does. The subsequent qualification explicitly prevents that extension. This is a clear pedagogical case of protective qualification (iḥtirās) because the threatened misreading is identifiable.
If no such misleading implication arose, a mere added negation would not establish the same device. The explanation must show what the first clause risked implying and how the second prevents it.