They're running a genocidal campaign against all beast tribes to exterminate Primals. They don't care who was even responsible for the summoning, as shown when we were introduced to the Vanu Vanu; The Garleans were ready to exterminate the Zundu just because the Vundu summoned Bismarck. The Zundu even helped us put Bismarck down... The Garleans don't care though; Vanu Vanu summoned a Primal, all Vanu Vanu must die.
That is something I found odd when it came to our encounters with him, but then again... He is the Legate of the VIth Legion... Nael was Legate of the VIIth Legion, and he summoned Meteor, and had a few other Sephiroth moments... Stands to reason that Regula will do something similar and reference Final Fantasy VIs villain, more so since he is stationed next door to the Warring Triad, which were central to Final Fantasy VI... I'd predict that he has plans to "copy" Ysayle, following that revelation... 3.2 Fiend Trial, 3.3 Demon Trial, 3.4 Goddess Trial, what does that leave for 3.5? A Kefka Trial, with Regula being our stand in for the clown... Would actually make what Elidibus said on the Gerun Oracles a bit funny... "Our fates were ordained long ago, Archon. The Garleans are no exception. Nor the Triad.", inb4 the Gerun Oracles is just the script for Final Fantasy VI...
Even failing that, how do you process knowing that people can summon Primals? You can't very well kill all people... No, they need to learn more first... Queue someone reminding us that Gaius was studying the Echo, and queue "All Echo users must die!", which was something the Garleans touched on in 1.0 for all of 5 seconds...
Ignoring potential speculation, I can't even blame him for that. Azys Lla holds the secrets to sealing Primals, that is a priority. For all he knows (and for all we know), Ysayle (and Thordan, though Regula seemed to be in the dark about all that) were the only people capable of this summoning. If it was something we were capable of we'd have used it to wipe the floor with him, and we didn't. It was something to report, I'm sure, but certainly not a priority. "People can summon Primals somehow!" is not as important as securing the means to seal Primals. Get a means to beat your enemy, before making more... Or antagonizing old ones, rather...
I'd like to think that whole mini-trial we had with Regula was him testing us, if we could whip out a Primal, we'd have done so there. The fact that we didn't makes the revelation somewhat moot, at least for the time being. All he has to go on is that someone who is now dead could summon a Primal. If I had to guess, I'd say they'll be looking into it, and that potentially opens up the doors for Nero to storm back into the plot, since Nero might bare some fruits of Gaius' interest in the Echo.