The fact it hit an empty seat is irrelevant and pure luck. It easy to justify excessive force when a player deliberately kicks a ball with that level of power into an area where his opponents are seated.
If you throw an object towards someone with similar intent it's easily justifiable as red, kicking a ball you know is out of play is the in the same ballpark for me.
If that happens in my game I'm going red card and I think that would absolutely be the expected outcome.
And as a fully paid up member of the "any contact wit ball out of play is excessive" brigade I am surprised you are on the other side of the fence on this one.
Of course these matters are in the opinion of the referee and If you don't see it as excessive or brutal, that's your choice, but I think games at our level would descend into chaos it we didn't sanction this correctly. Arguably, this one did, but there were other aggravating points in the game as well so I can't single this one incident out as the cause for that, although it was from this point things took a real turn for the worse