No it isn't. If they have no rule for it they can't suspend him.
If they don't have a rule, they still can.
Rule 28 states,
the Commissioner may, at his discretion, investigate any incident that occurs in connection with any Pre-season, Exhibition, League or Playoff game and may assess additional fines and/or suspensions for any offense committed during the course of a game or any aftermath thereof by a player, goalkeeper, Trainer, Manager, Coach or non-playing Club personnel or Club executive, whether or not such offense has been penalized by the Referee
"any offense committed", it does not need to be a codified rule AND no penalty needs to be called on the player, let alone a matching penalty.
Also, the Head of Player Safety's role is to make decisions in the name of the Commissioner.
But even still, the play can be penalized on at least three grounds: fighting, aggressing, roughing.
Fighting is left intentionally vague so that refs have more discretion and can be more judicious on the ice, but Rule 46.1 includes "at least one player punches or attempts to punch an opponent repeatedly or when two players wrestle in such a manner as to make it difficult for the Linesmen to intervene and separate the combatants" - Wilson threw punches and was wrestling with several players. This could apply to every other player as well, and I doubt you would ever see supplemental discipline based on that alone, but it's technically the rule, and you were looking a rule, so there's one.
Aggressor, Rule 46.2 encapsulates "throw[ing] punches in an attempt to inflict punishment on [an] opponent who is in a defenseless position or who is an unwilling combatant" - Wilson punching Buch on the ice is a pretty textbook example. Both Zach Kassian and Alex Burrows have been suspended using this rule during Parros' tenure as Head of DoPS (Burrow's suspension, quite notably, involved Burrows pulling Taylor Hall down to the ice, and punching Hall while he was on the ice).
Roughing, 51.1, includes "a punching or slamming motion with or without the glove on the hand, normally directed at the head or face of an opponent". Again, his punch on Buch is pretty textbook, and it was in fact the exact rule they used to fine him, so the play was
clearly subjectable to supplemental discipline. There have been 3 suspensions and 9 fines based on roughing since Parros became the head, so, again clearly, roughing is both a penalty that can be the subject of supplemental discipline, but also there is precedent for it to be suspendable.
Lastly, the CBA, specifically section 18.5, defines supplemental discipline as including both fines and suspensions and does not differentiate between the two as alternatives of each other, but instead clarifies them as extensions of one another. Discipline can be as follows: 1) no discipline 2) fine <$5000 3) fine >$5000 4) suspension <5 games 5) suspension >5 games. It is not like there are some actions or penalties that can only be applied to fines but not suspensions or vice versa - if Wilson could be fined for what he did, he could've been suspended for what he did.
DoPS made their decision, but it is not because what Wilson did was not suspendable, and in fact there is strong precedent for it and Wilson has a history that shouldn't be ignored. And
that is why people are taking exception to their decision; people feel they made the wrong one.
