Sure, there isn't really an enforceable non-compete in California, its one of the things that makes Silicon Valley, well Silicon Valley. Second there is a sort of hubris around 'the best revenge is beating them at their own game.' I know of at least two founder/co-founder pairs that split over different ideas about how the vision should go and rather than try to sue the other out of existence, the sense of invulnerability or desire for competition gives rise to a sort of 'bring it on if you got it' mentality.