The Catachans get away with it all the time
But, as a rule, the Cadians seem to have far less Commissars, period, than most regiments. And I can't really see something as much of an insult as attaching an off-world Commissar to a pure Cadian unit going unpunished either way. Especially not when that Commissar then threatens a highly decorated hero like Thade.
I mean, I may be reading too much into some of the Eye of Terror background and some of the brief snippets we've seen of Cadian life. But it's always seemed to me that they're the exception to the rule of the normal Schola Progenium situation.
Stormtroopers? Don't need 'em, we've got Kasrkin.
Commissars? Again, don't need 'em. The "Iron Discipline" rule was put into last edition's
IG codex and heck, the Cadian Shock Troops list in the Eye of Terror book had it too. It was supposed to represent the fact that Cadian officers undergo the same generalized training, but don't seem to favor the "executions to improve morale" idea that the Commissariat at large favors, instead seeming to favor the Gaunt methodology of "it's not ALWAYS a bad idea to withdraw, and field executions are wasteful".
But I have an undying love for the Cadians either way, so I might be biased