AFAICT, American copyright law does say "To prepare derivative works based upon the work" is not allowed, and doesn't say anything about private derivatives being an exception to that.
Preparation means getting ready for something. here, that almost certainly means redistribution.
You're not breaking the law if you scribble notes in the margin of a textbook; that's just crazy. Even if you pass that on to a friend, for that matter.
Anyway, if a license tries to rely on such draconian doctrines to prevent uses, it's obviously not a free license.
I'm guesssing that likely intent is so that the authorities could raid a large scale copyright infringment operation, and obtain convictions based on evidence of perparation alone. I.e. not have to catch anyone red-handed redistributing the prepared materials.
Kind of like how cops in some places in America can evidently arrest someone for DUI if that person merely walks to their car with their car keys, intending to sleep inside until sober.
https://en.wikipedia.org/wiki/Copyright_law_of_the_United_St...
Certainly if you privately modify it is going to be unlikely for someone to find out you did that, but that doesn't make it any less illegal.