The question is not whether Weinandy did something wrong publishing criticism of the Pope; it's whether it was reasonable to can him for doing it. In Professor Küng's case, he had already been canned, years before he published the criticism you refer to. (Although he wasn't exactly reticent with his criticism in his youth either.)

Professor Küng was retired from the faculty of Ecumenical Theology of the University of Tübingen. Criticizing churches is part of Ecumenical Theology. Weinandy was employed by the Catholic Church as a doctrinal purity inspector. Retired professors can't be fired; their pensions are paid by secular taxes. Speaking freely on controversial subjects is one of the things they are expected to do.

Weinandy is a member of a religious order. Küng is a diocesan priest in a diocese (Lucerne) which has a long-standing reputation for independence from Rome. It is not likely Küng's bishop had any objection to his letter.