Alien of the Week Features
The Sheliac government -- if not the species in general -- has little sympathy for other species. They appear to respect the Federation as a coalition with sufficient resources to resist them, so they signed an extremely detailed peace agreement. Nonetheless, when they detected human colonists on a planet granted to them under the terms of the treaty, they insisted that the colonists be removed before they arrived, otherwise they would exterminate the colonists themselves.
They are extremely legalistic, adhering to the strict letter of the law. They treat their treaty with the Federation as a rigid legal document that cannot be re-negotiated or re-evaluated once signed.
The planet in question was in a region of space saturated with "hyperonic radiation", which was harmful to humans and prevented the normal operation of some Federation technology, especially transporters and phasers. Since the Sheliac were planning to establish a colony there, it is reasonable to conclude that their own physiology and technology would not have been affected.