I have a question for you: What would you say if high ranking churchmen (even the Pope) began to teach things that were explicitly condemned as errors by previous Popes?
Would you a) say that we should accept that teaching which had always been condemned as an error; or b) resist that erroneous teaching by holding to what the Church has always taught, even if the said error is coming from the Pope Himself?
The SSPX has choseon option “B”. They are holding fast to what the Church has always taught, and resisting the errors that have been repeatedly condemned for the past 200 years.
When the Pope gives mixes signals, or explicitly teach what has been condemned, the SSPX simply holds to what the Church has always taught and resists the modern errors.
A few of these errors are: religious liberty, which was condemned repeatedly yet has been promoted by the past few Popes; and false ecumenism, which does not seek to convert heretics and schismatics into the Catholic Church, but rather seeks to “unite” with them without their converting.
Those are the two primary errors that the SSPX resists. Both of these errors were condemned before Vatican II, and both are now taught and promoted by all levels of the hierarchy.
The SSPX also reject the liturgy of the New Mas because it is has a Protestant flavour. Actually, it has more than just a Protestant flavor. It has incorporated most of the “reforms” that the heretics of the 16th century incorporated into their “mass” with the purpose of destroying the Mass. The Novus Ordo Mass is virtually identical to the service at the Episcopal church which I grew up attending.
The Pope is not garanteed to be free from all error unless he is defining a dogma. Thus, when the Pope promotes the two above mentioned errors (religious liberty and false ecumenism), Papal Infallibility does not enter in.
If you would like to read previous encyclicals where the two above mentioned errors are condemned, here are a few links:
Condemning false ecumenism:
papalencyclicals.net/Pius11/P11MORTA.HTM (the entire encyclical is dedicated to false ecumenism.
Religious liberty. This error is condemned in many encyclicals, The following is just one of them (see paragraphs 19 and 20):
papalencyclicals.net/Leo13/l13liber.htm