crossref, I think that the IRB think that that is what the law says - and it is how most referees would interpret the law. Though I have seen too many TV games where the kicker starts on the mark then takes two big steps forward before actually punting the ball - which should be called back.
Of course it is not what the law actually says:
Law 21
21.2 Where penalty and free kicks are taken
(a)
The kicker must take the penalty or free kick at the mark or anywhere behind it on a line through the mark. If the place for a penalty or free kick is within 5 metres of the goal line, the mark for the kick is 5 metres from the goal line, opposite the place of infringement.
(b)
When a penalty or free kick is awarded in in-goal, the mark for the kick is in the field of play, 5 metres from the goal line, in line with the place of infringement.
Sanction: Any infringement by the kicker’s team results in a scrum 5 metres from the goal line in line with the mark. The opposing team throws in the ball.
(c)
If a quickly taken penalty kick or free kick is taken from the wrong place the referee will order the kick to be taken again.
Everywhere else in the laws - e.g. in law 21.4, if there are multiple paragraphs leading to one sanction, the sanction applies to the section.
So if we apply the letter of the law, any kick taken from the wrong place after we have made the mark, we advance the scrum to 5m from the opposition goal line. Or is that retreat to within 5m of the offending teams goal line.
On the subject of what were the law makers thinking:
22.4
(g) Kick taken in the in-goal. If a player retires into in-goal to take a penalty or free kick awarded in the field of play and a defending player by foul play prevents an opponent from scoring a try a penalty try is awarded.
What, when and a side order of why?
The opposition need to retire untill they are 10m from the mark - so 15m from the goal line. Potentially 40m from the kick.
Once the kick is taken there is open play, so law 22 and law 10 cover penalty tries adequately, not sure why it needs to be mentioned here.
Also a free kick can be awarded in-goal. But that is not mentioned here.