"Fiktion" in a legal sense can mean "implication" and one of the online dictionaries I use translates "kraft rechtliche Fiktion" as "implied by law" in the sense that an opposing ideas are insignificant. As is typical when working with German compoiunds, it's a matter to put the two separate words together and (try to) interpret the result. I could be more sure of this answer if I had the text under the heading, too.