Good question! Since it is the Church's teaching, I'll let the Church answer it.
22. Actually only those are to be included as members of the Church who have been baptized and profess the true faith, and who have not been so unfortunate as to separate themselves from the unity of the Body, or been excluded by legitimate authority for grave faults committed. "For in one spirit" says the Apostle, "were we all baptized into one Body, whether Jews or Gentiles, whether bond or free." As therefore in the true Christian community there is only one Body, one Spirit, one Lord, and one Baptism, so there can be only one faith. And therefore, if a man refuse to hear the Church, let him be considered - so the Lord commands - as a heathen and a publican.  It follows that those who are divided in faith or government cannot be living in the unity of such a Body, nor can they be living the life of its one Divine Spirit.
So there are three conjuctive conditions, the last of which is divided into two disjunctive conditions: (1) baptized, (2) profess the true faith, and (3a) have not separated one's self from the unity of the Body, or
(3b) have not been excluded by legitimate authority for grave faults committed.
Sedevacantists and some independents do not believe that they "have been ... so unfortunate as to separate themselves from the unity of the Body" because they don't believe that the members of the present hierarchy are professing the Catholic faith itself, which, by this very teaching, is a necessary prerequisite for visible membership in the Church. This being the case, they do not believe that they have separated themselves from the unity of the body; rather, they believe that it is the Modernists (and all those who receive, defend or harbor them) who have separated themselves from the unity of the body by failing to profess the inviolable Faith.
The SSPX believes that its disobedience is lawful given the circumstances, but someone else will have to represent the SSPX since I am not familiar with how it views itself in light of the above-mentioned conditions.
EDITE: remove double negative, grammar, clarity