Perikles, I guess mine was not the best example but I'll try to give a plausible explanation.
In all the other sentences put forward we have "el cuento" defined, i.e. Le contaron un cuento pero no se lo tragó. Therefore the pronominal form is used and this is the most common usage, BTW.
Another example: No me trago lo que acabas de decir - I don't believe what you've just said. Again, there's a DO (what you've just said)
However in my sentence, I just stated that they tried to deceive him but did not specify how; it could have been verbally or physically. So, you couldn't you the "se" because the person would be literally swallowing himself, and you can't use the "lo" because you haven't got anything preceding to refer to.
Then again, it could just be colloquial usage and someone may rap my knuckles
