لِأَنَّ الصِّفَةَ الْمَقْصُورَةَ عَلَى الْفَاعِلِ مَثَلًا هِيَ الْفِعْلُ الْوَاقِعُ عَلَى الْمَفْعُولِ لَا مُطْلَقُ الْفِعْلِ
The attribute confined to an agent, for instance, is the act as performed upon its object, not the bare act in abstraction.
“To strike ʿAmr” is a more determinate action than simply “to strike.” When the agent is singled out, the object can be part of the very predication that is restricted.
مَا ضَرَبَ عَمْرًا إِلَّا زَيْدٌ
No one but Zayd struck ʿAmr.
Zayd is the restricted-to agent, while the relevant predicate is the whole action “striking ʿAmr,” not striking without qualification.