And - note - the section prohibiting a single stud at the toe is headed "Construction and design"
If the single stud is simply the remaining stud of a pair where one is lost the boot was neither constructed nor designed with a single stud...
Disagree, If it is a single stud and the front where there used to be two, YOU have constructed it in this way.
Take the other one out, or put another one in!
With regards to the designed single stud, may I turn the question around?
What advantage does the single stud give that they are designed that way? And also consider that in sports where this is allowed, the players have protection to avoid serious injury, where as rugby players do not.
I would suggest the issue is pressure, the pressure on a single stud is twice that on double studs. and as such is more likely to puncture the skin should it come in contact with a player. This is not saying double studs will not do this too, but the chance of all pressure being on one stud and not the other is much reduced.
If you want proof go to the beach and stand in the wet sand with your feet apart, this distributes your weight through both feet. you'll not sink very much. Do the same standing on one leg so all your weight is through that foot. The imprint you leave will be much deeper.