"Slavery" in this case is just a word. Janissaries were called "slaves" because they were the personal troops of the Sultan, under the Sultan's personal command (as opposed to the command of nobles and subordinates). The whole point of devshirme was to create a military and administrative elite that was personally loyal to the Ottoman state, i.e. the Sultan, and to prevent subordinate nobles from usurping power. In the later Ottoman Empire, the janissaries themselves became the most powerful force in the empire and installed/removed Sultans as they fancied. Can you show me any other instance where the "slaves" had more power than their own "master"?
Although most parents would not willingly give up their children to foreign strangers, and there are indeed instances of resentment in the Christian provinces of the empire, there are also cases where families voluntarily submitted their children into the system and bribed officials to let their children in. The opportunities provided by the Ottoman system of devshirme to lowly peasant children were unmatched elsewhere in Europe, such as the world-class education of the Enderun school (
Enderun School - Wikipedia, the free encyclopedia). Obviously, the Ottoman system of devshirme is morally unacceptable by modern standards. But can it really be compared, for example, to the Arab and European enslavement of millions of black Africans and their socioeconomic subjugation, or for that matter the relegation of entire communities to social damnation on the basis of "impure birth" in historic India? I think that is highly questionable.
Also, on the topic of the thread, the Jews were exempt from devshirme.