Why did companions and their successors accept monies and properties from oppressive rulers?

Acceptance of stipends or funds was quite usual as it is permitted in the shari'a as the funds are permitted and a right of Muslims.

The Sunni jurist Abu Bakar Ahmed bin Ali Jasas Razi (d. 370) records:
"Hasan Basri, Saed bin Jubayr, Shau'bi and all Tabayeen used to take stipends from oppressors, but not because they were friends with them or deemed their reign as legitimate, rather they used to take it because it was their own right which was in the hands of oppressors and Fajir people. How could this happen on the basis of friendship when they were confronted with Hajjaj via sword, four thousand Qura (scholars) who were the best and jurists amongst the Tabayeen fought against Hajjaj at Ahwaz under the leadership of Abdur Rehman bin Muhammed bin Ashas and then fought Hajjaj in Basra and then at the places of Deer Jamajam near Furaat in Kufa. They had broken their allegiance with Abdul Malik bin Marwan, they used to curse and do Tabbara on them [Ummayad rulers].
Prior to them, people had the same behavior with Muawiya when he became ruler after the murder of Ali (as). And so Hasan and Hussain & the companions (sahaba) of that time (also used to take stipends from Muawiya), they werent friendly to him, in fact they used to do Tabbarra on him [Muawiya] in the same manner as Ali (as) used to do (tabbarra) till Allah (swt) took Ali to paradise and Ridhwan. Thus, accepting the position of a judge and taking stipends from them [oppressors] does not mean that those people were on friendly terms with them or accepted their rulership." Ahkam al Qur'an al Jasas, Volume 1 pages 86-87.

The Shafii jurist Imam Ghazali observed: "There were many among the companions who lived up to the time of tyrant rulers and used to accept properties from them. Such were Abu Hurayra, Abu Sayeed Khodri, Zaid bin Sabei, Abu Ayyub Ansari, Jarir bin Abdullah, Anas bin Malik and others. Some of them received from caliphs Marwan and Yazid bin Abul Malik, some from the turant governor Hajjaj. Imam Shafeyi received once from caliph Harun Rashid one thousand dinars. Imam Malik also received them from different caliphs. Hazrat Ali said: 'Whatever a ruler gives you, he gives out of lawful things. He himself did not accept it out of greater sense of piety. When Imam Hasan came to caliph Muawiya, the latter gave him four lac dirhams which he accepted. These sages used to accept properties of tyrant rulers. The supporters of above opinion say that some of the sages did not accept out of great sense of piety. This does not show that it is illegal." Ihya Ulum-id-din, Volume 2 page 98.