لأن لفظ المملوك مطلق يتناول المملوك من كل وجه، والمكاتب ليس بمملوك من كل وجه
The word *mamlūk*, taken without qualification, can refer to someone owned in all relevant respects; the *mukātab* does not possess that legal status in every respect.
ولهذا لم يجز تصرفه فيه، ولا يحل له وطء المكاتبة
Therefore, some ordinary incidents of ownership do not apply to the *mukātab* or *mukātaba*.
In the historical legal setting, a *mukātab* had a contractual pathway to freedom, which altered the master's powers. The passage uses this to show that common terminology does not automatically imply identical legal consequences.
The general interpretive principle is that an expression tied to full ownership should not indiscriminately be extended to legally qualified or partial ownership.