Why did Almighty God say in Surah Ya-Sin, Ayah 72: (Rukūbahum) with an open Rā, and not with a ضمة (ḍammah) on the Rā?

Allah Almighty's saying: "From them is their riding beast and from them they eat." The general public read "rakūbuhum" with a fatḥa on the rāʾ (رَكوبهم), meaning "their mount," which is a "faʿūlah" form meaning "mafʿūlah" (a passive participle). Al-Aʿmash, Al-Hasan, and Ibn As-Sumayqaʿ read it with a ḍamma on the rāʾ, as a verbal noun (maṣdar). This is a rare recitation, though it has a linguistic justification.