That's the same thing I was thinking Lu. Since both connectors can be interchanged the chance of a misconnection between the "side-stick" and the controll surface is fairly greater than when both are marked. The incident happened a few years ago so I suppose Airbus did a recommendation to all the airlines already operating the aircraft and took some action to prevent this kind of mishaps in the future.
I found this site which explains the principles of "fly-by-wire".
Airbus "fly-by-wire"
Marcel_MPH