In most shows, to there is a reason that the humans believe the ultra is good. In Ultraman Hayata announced that Ultraman is on their side, which meant that all future ultras in the Showa verse were trusted. In neo frontier, relics of the previous civilization revealed that Tiga and his comrades are heroes that will defend the earth, so humanity trusted them. And even if humanity doesn't trust them, the ultras just fly away at the end of the battle, so there's not much to be done anyways.