[B]y what rationale are we bound to interpret facts about somebody in the best possible light?
By the same rationale that causes us, in establishing guilt, to rely upon the presumption of innocence of which this basic rule is but a corollary. And yes, I'm being awfully prescriptive.
Considering the fact that very few of us are saints, giving someone the best possible benefit of the doubt is almost certainly giving them too much credit.
That may be so, but the risk of not giving them too much credit is much outweighed by not giving them enough, for few of us are demons either. Colour me a lunatic if you will.
One the greatest hazards of breaking the rule is exposing oneself in public as an ass [Asinus sp., not arse ... well arse as well.] A common example are the fools who shoot off in anger at a statement that seems to them ridiculous before asking themselves, "could this possibly be irony?" But in general, if you make no honest attempt to understand what someone actually might mean , most especially when it seems at first absurd, but instead read them through the lens of presumptive guilt (which lacks for evidence just as much as innocence) you are systematically depriving yourself of even of the possibility of understanding them.
Similarly one ought not begin a conversation upon the presumption that one's interlocutor is less intelligent, less educated, less reasonable, motivated by less honestly motivated &c. where evidence is not yet available to make any of these out.
In particular allegations of wrongdoing, such as fraud, ought to be supported by evidence. If you see someone making a factually incorrect statement, and nothing else, a reasonable person will infer ignorance rather than deception. There being a lot of ignorance to go round.
However, these are rebuttable presumptions and the longer a conversation continues, the narrower the focus of the light becomes. More than once I've been tempted to write something along the lines of "common decency requires me to view your statements in the best possible light ... but you are making that task very challenging indeed.."
And if there are obvious potential ulterior motives present
The same thing that may motivate us to outright dishonesty might motivate us to believe some disinformation we have received, no?
AC wrote "And yet, James Madison also signed, along with all the other Founding Fathers, the first Bible printed in new nation, which also bears a note by the founders that it was to be used in the schools." You think it beyond reason that they may hold this earnestly to be true?