There isn’t one, it’s free will, though I suppose mortals could be used as disease vectors to get to a more powerful entity. A Nemfected Whampire, the closest monster to a human, would probably lose the True Love Protection restriction. Lara therefore isn’t Nemfected. With Wizards its probably losing the power lost on swearing on their power and breaking it, or being able to cross a threshold without losing their power Eb asks to enter Harry’s temporary apartment. It strikes me that the domestic bliss of Thomas Apartment, with Justine and a baby on the way mean’t that his apartment had a threshold, and a normal wizard couldn’t enter or push through a spell into the apartment without drastic power loss.
Suppose Thomas Apartment was being survielled by a wizard because it originally didn’t have a threshold, but one started forming when the baby was conceived? and a family came into being. That would reduce the surveillance a wizard could do and they would have to resort to more mundane methods. That may explain the lack of magical residue, and the need to attack the main Raith residence (no threshold) instead for information despite the heightened risk of being found out.