It’s because the ‘Proving Loop Constructs’ are part of their live fire testing grounds, this is while not clear, present as is. You enter a Loop Set, and you are entering a live fire destructive testing ground. It is as simple as that. The Trigs that exist there do so willingly, and to some degree it seems that Triglavian Society is some level of ‘post-death’, while they will try to avoid deaths of their own where possible(probably for resource reasons) death doesn’t seem to be permanent at a broad level for Triglavians.
Point of specific, the Loop Construct is also not where the Trigs actually live, but the majority of Trigs do still live in the Abyss.
That said, most NPCs in-game do not respond to standings even if you have good ones with their faction. That’s only a fairly recent change with specific NPC types that has nothing to do with faction and instead with what kind of NPC is being used.
For instance, the NPCs in the current winter event sites don’t give negative standings drop to Sansha.