That's a strange claim, Sagi. Why do games -have- to allow choice? Especially with a fixed protagonist?
Games don't have to allow choice, but it's a thing that defines them as a medium - especially RPGs - Interacvtivity. And in Witcher it's one of the biggest reason it's so great.
Example - (simplyfing) in a Witcher games, Geralt has a choice to save someone, or leave him getting killed.
In books you only have one path to go - the one imagined by the author.
More wide choices, make a good character design very hard, because you are playing a different Geralt every time - its gets convoluted and the character often is not as fleshed out as the same character from the book or movie.
Geralt in the books is more fleshed out than in the games - but that's a compromise you have to pay when designing a game - and it works very well in Witcher, because the choices you make, all fit Geralt's character (fixed protagonist). And the characters are still very well designed and complex.
It does't work that well in games with a custom character - Skyrim, Mass Effect for example. The main characters are just very simple and bland (in my opinion). Your choices are so wide ranged - even the sex, race, evil or good - that characters become simpler and simpler (even the side characters).
It's next to impossible to make a game like Skyrim with that quality characters as the Witcher.