They just don’t get it. Acts that are perfectly legal and normal become overt acts in furtherance of a criminal conspiracy when they’re part of the plan to commit a crime.
@georgetakei To be fair, it takes a lot of first-year law students a while to come to grips with that point.  At least, it did in my Crim class. My prof's explanation, which I found helpful, is that there are LOTS of things that the prosecution has to prove, "elements of the offense" in Legalese, that are not in themselves criminal. For example, neither driving a car nor being drunk is any more illegal than anything on that list; the crime consists of doing both of those perfectly legal things at the same time.