If an insider knows something materially bullish and non-public, exercising options and holding the exercised shares is the only legal way to exploit such knowledge to increase one’s effective equity stake. This, above all else, is what makes such a transaction a clear bullish signal.
p.s. I reject biomaven's contention that executives and their financial/legal advisors are too incompetent to know what they are doing.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”