Keep in mind that slavery for these people was nothing like the chattel slavery of the US south. Being an educated slave did mean that you had some restrictions on what you could do; it also freed you from having to worry about food and shelter, say. Similar tradeoffs have been made by salaried workers in Japan in the 70s and throughout the Western world during the industrial revolution
And even the restrictions part was ... varied. If you're a slave and your owner lives in Athens while you live in a Greek city on the Bosporus and supervise your owners business interests in the Black Sea (an actual example I recall from some of the primary sources from my Greek history classes in college), you really don't have much in the way of restrictions on you. Yes, legally you're a slave. So what? It has very little impact on your day-to-day life, at least as long as business is good.