Hi,

the document you see before downloading the game, says it is not allowed to cheat in this game? Afaik this is a singleplayer offline game, does CD Projekt Red really don't allow me to use a trainer with god mode or something on a singleplayer game??

There's probably a 100% chance this rule will be utterly and completely ignored :)
Also fairly certain files will be opened and reversed as necessary in order to do modding. Last time that happened nobody got in trouble. People were actually hired for their work..

Skyrim legal says you arent allowed to modify it, thats not enforced obviously

Agreed, I was just as baffled as you are. Cheating has always been considered OK in single player games, even to the point that most games in fact included cheats.
Installer for Witcher 1 has simple EULA that says nothing about banning cheating, I see no reason for W3 to behave differently.

And if this is about unfairly easily gained achievements, then that should be addressed in another way, not with legal threats. Heck, people know that on Steam, getting achievements is easy anyway, in CS:GO many can (and are) be gained by abusing certain servers.

To those saying that this is only a formality that won't be enforced I say this : It is in the license and can be enforced. Just that it most probably won't be enforced does not mean that it cannot be enforced at some time in future. Say ïn the unfortunate event that CDPR is bought out by EA.
I cannot in good conscience install the game on GOG Galaxy only to in turn (perhaps) violate license (by messing with game memory to increase money/HP to remove need for mindless grinding) which I agreed to uphold.

I think this term was implemented due to the following options:

A) CD Projekt RED just copy-pasted a part of the agreement from another game, because they haven't finished the final version of the terms of agreement and they just needed to present some form of it to the pre-download clients

B) They don't want people to bumrush the game on day1 with trainers/cheats/hacks/whatever

C) They are just messing with you

A = Quite possibly. Wouldn't be the first company to use C&P TOS from other games, or from a standarised form that later gets modified for the specific game.

B = Will happen regardless

c = unlikely