16.3 (b&c) are listed as dangerous play. 16.3(d) isn't, I can only presume this is deliberate.
I think it simply goes without saying that jumping on a ruck is dangerous
The fact that "dangerous" is mentioned repeatedly in law, trumps that suggestion
I agree. Furthermore
16.3 (c) - collapsing the ruck
17.2 (e) - the maul equivalent of 16.3 (c)
20.9 (a) - the scrum equivalent of 16.3 (c)
All have a counterpart in Law 10.4
[LAWS]10.4 DANGEROUS PLAY AND MISCONDUCT
(k) Dangerous play in a scrum, ruck or maul. The front row of a scrum must not rush against
its opponents.
Sanction: Penalty kick
Front row players must not intentionally lift opponents off their feet or force them upwards
out of the scrum.
Sanction: Penalty kick
Players must not charge into a ruck or maul without binding onto a player in the ruck or
maul.
Sanction: Penalty kick
Players must not intentionally collapse a scrum, ruck or maul.
Sanction: Penalty kick[/LAWS]
However, the restrictions for jumping on top of the ruck - 16.3 (d) and the maul - 17.2 (f) -
have no equivalent reinforcing counterpart in Law 10.4, therefore I can only conclude that jumping on top of a ruck or a maul is not considered dangerous play. It must therefore be related to something else, and my guess would be that it is related to killing the ball.
In fact almost all mentions of Dangerous Play in the Laws of the Game has a reinforcing counterpart on Law 10 4
Here is a list
16.3 (c) collapsing a ruck > 10.4 (k) line 4
17.2 (e) collapsing a maul > 10.4 (k) line 4
20.1 (i) charging while forming a scrum > 10.4 (k) line 1
20.8 (h) collapsing a scrum > 10.4 (k) line 4
20.8 (i) lifting opponent in scrum > 10.4 (k) line 2
20.9 (a) collapsing a scrum (10.4 (k) line 4
The only ones that are not mentioned are...
16.3 (b) kneeling in a ruck
6.B.4 (b) Assistant referee flagging Dangerous Play
Interestingly, there is no Law that
specifically states a player cannot jump on top of a scrum, although in practical terms the SH is the only one in a position to do so (all others would be offside first) and in any case, that would seem to be ruled out by
[LAWS]20.1 (e) Number of players: eight. A scrum must have eight players from each team. All eight
players must stay bound to the scrum until it ends. Each front row must have three players
in it, no more and no less. Two locks must form the second row.
Sanction: Penalty kick[/LAWS]