Why would anyone badmouth crap in such a way.. if we kept eating and the crap just built up inside us we would live very short lives.. and grow super fat super fast..
More on topic: The context I read into this is that the person has already said that crap = crap or that they don't like to crap or something of the sort. And they are further explaining why it is so. And that is usually the sort of situation it is used in. A statement is made without mentioning the reason and when challenged, you use datte. That's my impression at least.
A:あした学校さぼろう!
B:はあ!なんなんだよ!
A:だって、つまらないから!
It may just be me having watched too much drama/anime, but does it give sort of a childish impression? As if the argument is not very serious? Or maybe it implies that the person very emotionally invested?