According to several people, it absolutely is in pretty much every coach's contracts. The difference is, that nobody else will enforce that after firing the coach, because they want to get out of that salary obligation. Vegas on the other hand is doing it, either because they really want to take advantage of a competitive edge by keeping Cassidy from working, or the owner has a big personal issue with Cassidy and wants to keep him from working.
And again, they don't submit these contracts to the league. The contract between the coach and the club is only between them. This is different from the players, who have a CBA, and they are basically contracted to the league, not to the individual clubs.
If the NHL isn't involved then why did Edmonton ask them to get involved and the NHL approve what Vegas is doing?
There has to be a limit to what's in a contract that the NHL is willing to tolerate.
For instance, do we really think that if some coach got all the owners drunk and talked them into making them the head coach of all 32 teams, the NHL would just be like, "sorry we don't have oversight over coaches contracts."
Of course not. They would find some mechanism to prevent that, or levy a punishment enough so the teams didn't follow though with it. Citing some kind of "anti competitive" or "impact on the integrity of our league" or "reputational harm" argument.
They could threaten the team to make them forfeit all their draft picks for 20 years. That's something they can do and no court can step in to tell them how to run their league. Plenty of other ways, as long as it's not financial. They can prevent them from holding any league events or their players from playing in the All Star game indefinitely. They can ban them from GM meetings. They can cease all league promotion of the team and their players. They can do lots of things.
They need to make sure things are being run on the up and up in their company, and as you state, this is being done, "either because they really want to take advantage of a competitive edge by keeping Cassidy from working, or the owner has a big personal issue with Cassidy and wants to keep him from working."
Those are not legitimate reasons. That's a team with a history of shady behavior, acting like they can do whatever they want, and opening the league up to lawsuits, and negative PR that could affect their revenue, and TV deal negotiations, for allowing it.