I mean, they are and use their diplomatic channels to express that point. However they were also in the wake of a war that tore through to Macedonia, from Bavaria. That isn't a small amount of suffering the Romans took. The Roman people more or less demanded a punitive campaign before the war was over, because outside of the Triunes, the Holy Roman Empire was the next, if not the bigger, geopolitical threat, and rival. We can't reflect on the sentiment as if that wasn't the case. Roman territory from Greece to Italy had been set on fire.
Whilst yes, there have been geopolitical repercussions to this punitive campaign, there have also been rewards. There will be a fight to control Germany, which is a complete flip from the start of the war - that fight only really happening because the punitive campaign kicked the legs out of the metaphorical unstable chair. The Romans have gone from sitting still in Europe to dictating terms in the East, and with Hungary on their side in that punitive campaign, they've roundly ensured a wall of buffer states that have geographic protections. (I admit I'd rather the Romans held them, but what can you do.)
The Triunes are a threat in a number of ways - one they bankrolled the Madmans War (AKA Theodor's War) and that ability to bankroll others to fight the Romans is a threat. They are in conflict in India, sure, and are set to become more economically powerful as a result of the New World.
They also were behind raids in Roman ports for no discernible reason. They attacked the Romans with no declaration of war, with no provocation, all for the sake of loot. There is a score to settle there.
Further, and I think this is a slight irony given that Arles really was one of the earliest points of conflict for the Romans and Triunes diplomatically. Arles doesn't want to be Triune, the Romans have been their ally for ages. If not for that, who knows, the Romans could well agree to terms with the Triunes, if not for the history of looting of Roman ports by the Triunes.