Mugendramon

Whatevs, Tasigur was just a non-izzet commander. If it didn't count for the color identity you would have run it in any color combination that used either U or R. That's a lot of potential splash. Nobody's denying some companions are good, that's for sure. Lutri in particular was just too blatant an include in anything that could play it with 0 deckbuilding cost that they just didn't want to let it loose.

In the sense that you can't play it in tasigur like you could if it didn't count

Fair, loses a bit of splash then

The reason for the ban is basically how it bears zero deckbuilding cost. It even exists outside of the game and is unbound by commander identity- you needn't izzet colours to play it, just being able to cast it. The companion requirement being playing a singleton format, it would mean every deck would like to have it as an extra option which doesn't even eat up a deck's slot as it doesn't count toward the 100-card total. Zero deckbuilding cost and absurd splashability equals instant addition equals undesirable card in a format.