My understanding is that the poachers aren't necessarily accepted by the pact, but they are subject to the pact, since they are within the Hedge. So they didn't agree to follow the pact, but what they do can cause things like a Greenwrath (which those who do follow the pact can be affected by).
As to why the Ala Mhigans weren't accepted in Quarrymill, we can either take the Hearer's word for it (e.g. "The Elementals said there is too much hatred in his heart"), or we can note that the same Hearer gives us some completely spurious leve quests to destroy Gelmorran ruins that a pair of Duskwights have come into the area to explore. Note that the same type of ruins exist all over the Twelveswood, and it's only here that, somehow the Elementals are offended.
Then note that nothing in lore, quest text, or anything else says that Hearers must be honest or benevolent.
As to why the Ala Mhigans weren't accepted in Quarrymill, we can either take the Hearer's word for it (e.g. "The Elementals said there is too much hatred in his heart"), or we can note that the same Hearer gives us some completely spurious leve quests to destroy Gelmorran ruins that a pair of Duskwights have come into the area to explore. Note that the same type of ruins exist all over the Twelveswood, and it's only here that, somehow the Elementals are offended.
Then note that nothing in lore, quest text, or anything else says that Hearers must be honest or benevolent.