They do this for the same reason they put out their fake experiences at a$$emblies or yearbook. This is to create the illusion that Jehovah really protects people--what about the witlesses that die in accidents because they went out in field circus while too tired or it was too dangerous to be out there? And, what happens when the witlesses get older and, because they wasted time pio-sneering, their retirement is crappy? I think they are pretty much sunk.
I could return a large sum of money because it's the law. In the US, for instance, it is the law that if you find money of $20 or more, you are to report it. If you simply wish to obey the law, you would report the money and, if possible, return it (obviously, if it's less than $20 and you cannot reasonably identify who belongs to it, it's yours to keep). Suppose I find $50,000 and honestly return it to its rightful owner. Upon interview, I identify myself as a Devil worshiper and that Satan wants people to enjoy the rewards for the value you create for society. Returning such money would allow its rightful owner to enjoy such rewards, despite a moment of carelessness or an instant of bad luck that caused its loss. I don't think that incident would ever be truthfully reported in the Washtowel rags.