I don't believe that is true. Janissaries were non-muslims (primarily christians) given a chance to earn privileges not otherwise available to non-muslims in the Ottoman Empire, and eventually became more privileged than the Muslim portion of the military (which lead to the decay of the Empire and loss of significant amounts of territory, but that was a bit past the middle ages when Janissaries started becoming that influential).
Relevant info from
http://en.wikipedia.org/wiki/Janissaries:
The Janissaries were kapıkulları (sing. kapıkulu), "door servants" or "slaves of the Porte", neither freemen nor ordinary slaves (Turkish: köle). They were subjected to strict discipline and were the first army to wear a uniform, but were paid salaries and pensions upon retirement and formed their own distinctive social class. As such, they became one of the ruling classes of the Ottoman Empire, rivaling the Turkish aristocracy. The brightest of the Janissaries were sent to the palace institution, Enderun. Through a system of meritocracy, the Janissaries held enormous power, stopping all efforts at reform of the military.
According to military historian Michael Antonucci and economic historians Glenn Hubbard and Tim Kane, the Turkish administrators would scour their regions (but especially the Balkans) every five years for the strongest sons of the sultan's Christian subjects. These boys (usually between the ages of 6 and 14) were then taken from their parents and given to Turkish families in the provinces to learn Turkish language and customs, and the rules of Islam. The recruits were indoctrinated into Islam, forced into circumcision and supervised 24 hours a day by eunuchs. They were subjected to severe discipline, being prohibited from growing a beard, taking up a skill other than soldiering, and marrying. As a result, the Janissaries were extremely well-disciplined troops, and became members of the Askeri class, the first-class citizens or military class. Most were non-Muslims because it was not permissible to enslave a Muslim.
So they were kidnapped as kids and did not have the possibility to leave the Janissaries, which makes them slaves. However they were paid and held some political power.
This covers the medieval period - past 1683 turkish muslim men could also enroll voluntarily.
