Modifying a program is illegal under copyright law, so yes you do need to read the license before doing that.

No, copyright law doesn't say anything about privately modifying a work. Only about distributing derived works.

It would behoove you to check the license in order to confirm that whoever gave it to you had the right to do so.

Beyond confirming that you have a legit copy, you don't have to be concerned with the license at all, if you're not redistributing anything.

The vendors of proprietary software and their lawyers dreamed up this idea that a license can lapse based on the user's non-redistributing uses of the work. For instance, if the user reverse-engineers the binary code to understand how it works, then they become unlicensed, the idea then being that they are perpetrating copyright infringement by continuing to have a copy.

The AGPL falls into this category.

The restriction itself does not come from copyright law; copyright law doesn't say things like that allowing network users to interact with a software program is an infringing activity, or that reading the work to understand it is infringing activity.

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.

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.

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.

That is definitely not what the word prepare means in that sentence. Its more like the meaning in the phrase "prepare a meal".

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.