'but' is used here because it is a quick and simple way to make a contrast. We can remove one 'because' like this: People don't spend hours looking at pornography on the screen for the reason that they are bad, but because the porn is so very good... (this sounds long and convoluted this way without the extra 'because') Observe it in another sentence: I don't eat apples for the taste, but because my doctor tells me to. I don't eat apples because they are tasty, but because my doctor tells me to . Or we can do away with 'but' and 'because' like so: I don't eat apples for the taste. I eat apples on my doctor's orders. (Now there are two separate sentences without the contrasting conjunction 'but'.) Hope this helps...