Free on the shock it can slightly move up and down, juste as Martin explains in the video... if you need preload, it means that you are not using proper springs. By the way, I'm used to touring car on carpet and asphalt, and absolutely not a specialist in rally :-)
In modern touring cars, the effect on the camber gain is quite negligible compared to the changes to the roll center...
Also an interesting video there:
https://www.youtube.com/watch?v=L6XNDw7Ql84