Because Big Businesses are much more likely to try to send out viruses than single programmers who have nothing better to do... (Not talking about BlueMonk, of course, but malicious ne'er-do-wells in general). I would personally trust downloads from a big business (i.e. Adobe, or Microsoft) much more than something I got off of somebody's homepage. Mostly that's because it's harder to hack a corporate website, and there are measures in place to ensure that the file you downloaded is the one the provided (i.e. Compare an MD5 Hash of the file)