If you look a little bit closer into camlistore, you'll see that it splits content with content-defined chunking, ie when there's a change in the middle of the file only this zone (camlistore targets 8kB) is really added to camlistore; there's an indirection between the file as an object and the actual content. That means that hundreds (or even thousands) of slight changes to the same big file won't change much.
Regarding misclicks, for camlistore everything is private by default; in order to make something public you have to build an authorization, which you give to someone. You also have the possibility to remove that authorization, and the data won't be publicly accessible anymore.
Immutable is an interesting idea - it's a lot less interesting when 100 different copies of the same slightly changed RAW file from my digital camera are using up 100s of gigabytes. Or I misclick and something goes into the public store which shouldn't - I might not be able to get rid of all of it, but I should be able to undo it a little.