Muhammad bin Al hasan Al Mukhzumi said “The sentence “that which is not reached by the Camel hoofs” means that the Camels eat (the arak trees) within the reach of their heads. So the land (where the arak trees are growing) may be protected beyond such a region.
حَدَّثَنَا هَارُونُ بْنُ عَبْدِ اللَّهِ، قَالَ قَالَ مُحَمَّدُ بْنُ الْحَسَنِ الْمَخْزُومِيُّ " مَا لَمْ تَنَلْهُ أَخْفَافُ الإِبِلِ " يَعْنِي أَنَّ الإِبِلَ تَأْكُلُ مُنْتَهَى رُءُوسِهَا وَيُحْمَى مَا فَوْقَهُ .
AbuDaud
Tribute, Spoils, and Rulership (Kitab Al-Kharaj, Wal-Fai' Wal-Imarah) • Hadith 3065