When did selling shares (especially when you have many) become dumping? As I recall he's doing his humanitarian bit nowadays, which strangely enough costs money.
Also, Steve Jobs (until recent health issues) was very active in every aspect of Apple, whereas Bill Gates has been in the process of disengaging himself from Microsoft.
You don't see Steve Jobs "dumping" his shares because he doesn't donate most of his money, like Bill Gates does.
You're completely correct. There's many reasons to "dump" shares, and very few of them are indicators of something bad.
He's playing a real world game of Pandemic with the resources of basically a man and it sounds like he's getting somewhere.
Since when is 13% a whopping number? Cut that out of a pie chart and you barely have a Pacman.