That's not the default in all Linux desktops I've seen.

There are two clipboards in X11. One is the middle-click-paste, selection clipboard that gets clobbered when you highlight something. But C-x, C-c, and C-v (in CUA applications) operate on a different clipboard altogether that doesn't exhibit this behavior.

clipboard and selection buffer are separate, though I think there is a situation that co-mingles them.