Apparently, long ago +ABL, +Williamson and other of the learned 'superiors' of the SSPX (and plenty of non-SSPX trad clergy) determined that when done "by the book," the NO ordinations and episcopal consecrations are valid. At the same time, they must realize that it is with very good reason that "we the people" have grave doubt and totally distrust the validity of all the NO sacraments - we especially doubt NO ordinations.
As such, what the SSPX really should be bound to do, is to *prove and publish that proof* of certain validity to all of the people in the pews. This should be an essential part of the welcoming into the fold of every single "convert cleric" that they use. If validity cannot be proven and broadcasted publicly, then they should conditionally ordain and publish that conditional ordination info to the people.
See, it's so screwed up because as you say, historically the Church always initially presumed validity, but I think that must change when it comes to NO ordinations thanks to everything and anything connected to the NO having been adulterated and corrupted.
So, Bishop Williamson once explained why he did conditionals when he felt that the Rite was valid, and that was because he realized that other well educated, intelligent, and reasonable bishops and priest had in good faith concluded otherwise, so he considered his opinion just that, his opinion, and did not want to impose it on others' consciences. I would adopt the same attitude. Just because I myself believe something to be valid, I would not impose that opinion on others' consciences. In other words, he felt that there was an objective positive doubt due to the fact that not a few serious people had come to the conclusion that they're either doubtful or invalid.
But, in the early 1980s, of course, there was the notorious case of "Father" Stark, who refused conditional ordination, and that precipated the split with "The Nine". Recall that in the early 1980s, however, Archbishop Lefebvre was very much seeking a practical arrangement with Rome, pleading to be allowed to make the "Experiment of Tradition", so ... much of his rhetoric frome that time period is precisely what +Fellay uses to back the same thinking from the neo-SSPX. Of course, he realizes that most people will not look at the "date" on a quote but just assumed that, well, if +Lefebvre said it, that was his position. Then of course Resistance will pull out quotes from when +Lefebvre had the opposite view.
But after the "excommunications", SSPX were of the mindset that they could hardly care less what "Rome" thought and didn't care if performing conditional Ordinations ruffled their feathers.