People have families, mortgages, debt, etc. Sure, these people are probably well compensated, but it is ludicrous to state that everyone has the stability that they can leave their job at a moment's notice because the boss is gone.
They didn't actually leave, they just signed the pledge threatening to. Furthermore, they mostly signed after the details of the Microsoft offer were revealed.
The safest option was to sign the paper, once the snowball started rolling. There was nothing much to lose, and a lot to gain.