Because, unless there is a standard or they are explicitly claiming it is unmixed, being mixed with something else isn't legally deceptive. (Nor is it being rancid, though the defective product liability / warranty of merchantibility may come into play there.)

The 2 extra-virgin and one "pure" avocado oils here that are apparently just soybean oil probably at a minimum aren't listing soybean oil on their ingredients list.

Sure, and I am not saying that some of them aren't covered under current law, just that the set of them that are is narrower than the set that people seem to think (and I would agree) are problematic, and that even for the ones that are covered dealing with them is far less straightforward than with identity standards.