Why does Frederick call Mr. Auld's forbidding his learning how to read "invaluable instruction?"

Because he unwittingly gives him the key to mental freedom, at a moment Frederick states "if you teach that black person (speaking of myself) how to read, there would be no keeping him. It would forever unfit him to be a slave." Once he knew the world that Mrs. Auld taught him, it was imposible for him to be satisfied being a slave.