Why did the Umayyads prevent people from converting to Islam?

G.R.Hawting, a Senior Lecturer in the History of the Near and Middle East at the School of Oriental and African Studies, answers this question in his book "The First Dynasty of Islam - The Umayyad Caliphate AD 661–750" by saying: "The problem for the Umayyads was that they had come to power as leaders of a conquering Arab elite and to have allowed the conquered peoples to enter Islam en masse would have abolished or at least weakened the distinction between the elite and the masses. The crucial privileges of Islam, from this point of view, were in the area of taxation. In principle the Arabs were to be the recipients of the taxes paid by the non-Arabs. If the conquered peoples were allowed to become Muslims, and to change their position from that of payers to that of recipients of taxes, the whole system upon which the Umayyads depended would collapse. But as the pressure from the non-Arabs built up, and the universalist notion of Islam became stronger, this problem became increasingly urgent for the dynasty and played a major part in the generally negative attitude of Muslims towards the Umayyad dynasty."  (p. 5)