SSPX sacraments are not invalid, they are illicit. “Invalid” means that it is not a true sacrament; “illicit” means it is valid but that the one who celebrates the sacrament does so unlawfully (without proper permission).
The SSPX claim that they are in union with the Pope. Therefore they are under his authority, and the Pope says that the SSPX currently are not allowed to celebrate the sacraments. They do so anyway in defiance of his authority. Therefore, the sacraments they celebrate are illicit (i.e., unlawful).
The Eastern Orthodox do not claim to be in union with the Pope. Their churches maintain the lines of apostolic succession necessary for valid sacraments, but they do not recognize the Pope’s jurisdiction over the universal Church. The Catholic Church recognizes Eastern Orthodox sacraments to be valid because it recognizes the valid lines of apostolic succession that have been maintained in these Orthodox churches. The Catholic Church does not make an issue over whether those sacraments are celebrated licitly for a number of reasons, including the fact that to do so would hamper efforts made at repairing the relationship between the Holy See and the individual Orthodox churches.
Basically, if the SSPX claims that it is in union with the Pope, then it has no grounds upon which to protest the Pope’s authority to declare that the SSPX currently is not allowed to engage in ministry to the faithful.
