How would the literacy of the powerless threaten the power of those in control?

Educated animals would see the injustice with a skillset that would enable them to organize and usurp the leadership that oppresses them. Education is the key to get out of servitude. Slave owners did not want their slaves educated for this reason. Napoleon did not want "lower" animals educated for this reason. Only the elite pigs were educated in order to maintain their oppression on others.