To me that seemed like a dumb move... why not keep them easily swappable? Even if not for the fast swap station, for ease of replacement or service.
I really hope that Tesla is reading this cuz I'm sure they haven't considered any of these great ideas.
I’ll throw in a suggestion... maybe their battery R&D the last 5 years has given them confidence that the new 3 batteries are stable and have longevity; since the S was all new they hedged their bets making the battery easily swappable just in case. And now with the 3 not needed and quicker/cheaper to manufacture this way. This is complete speculation.
To answer @McHoffa directly: Tesla is dealing with multiple trade offs and they are making their decisions using far more information than us plebes out here.