A common belief is oppressors are bad and the oppressed are good. This helps to perpetuate the cycle of oppression.
Superior virtue attributed to the oppressed begins when the oppressors lose their powers. The idealization of the victim is useful because virtue is good, subjection makes people virtuous, so it is kind to refuse the victims power which will destroy their virtue. If being rich is bad, it is a noble act to keep the wealth from his poorer neighbors.
Sooner or later, the oppressed will argue that being virtuous is a good reason for having power. The oppressed will use the same weapon against the oppressors.
References
Bertrand Russell. (1937). The Superior Virtue of the Oppressed.