That light is totally redundant. The white-on-blue right arrow as well as the solid line in the middle of the street already mandate that only right turning is allowed there.
Someone else suggested a great programmer would have changed the rules, but that would have just made the rules more complicated.
This solution is neat, because it keeps the basic rules simple while solving the problem. And to most people, they will never notice it is a problem.
I don't know about such a rule. There are some intersections in Germany where there are red and orange lights, but no green for e.g. just one direction. At such a place, when no light is on, rules according to the static signs apply, while other directions still show some light. So what you're saying doesn't hold at least temporarily there.
> This solution is neat
I don't think so. Someone unfamiliar with the place might think the light will turn eventually green, even if it's unclear where one could go if it did. It will at least cause confusion and unnecessarily draw attention away from actual traffic. And of course there's the maintenance and power cost.
So yes, there probably is a legal obligation to keep moving. If you went there and stood in front of the light for an hour a friendly but stern police officer will — legally, and within his mandate — instruct you to move along.
In case anyone is unfamiliar with the concept of public nuisance as a crime. It's pretty simple, "things that cause significant enough trouble for other people". Its the common sense escape hatch option for law enforcement being unable to see any other crime.