Without the signs, someone would simply arrive at the intersection and wait god-knows-how long for it to turn green, until they gave up and went right :)
The reason for the light is that according to regulations, at an intersection, every road leading to it has to have lights. You cannot have three streets with lights, and one with just a sign saying "stop and yield".
I still see this as the only way to communicate correctly the rules of the intersection. And yes, add a sign as well to let people know they can only turn right.
The thing is, in Europe, you have priorities in traffic - first the lights, when those are off/unavailable - the signs, and finally "the one on the right goes first".
The rationale behind this intersection is that you need all the vehicles obeying the same set of rules. So you can't have one car approaching the intersection obeying the signs (because no lights on that road), and another car obeying the traffic lights.
According to the maintenance mentioned in the original link, the other colors are still there...
(Google streetview shows it too, but the picture dates back from when the red light was still youthful twenty years young)