Stock options vest in the future if you are still working there. The idea is that it keeps you committed - otherwise everybody cashes in on the day after an IPO and walks, and it rewards people who were the reason for the success.

You can't leave and then expect to get unvested options = otherwise people would simply sign up for every startup, stay a month and move on - then come back years later when the company is a success and ask for their million dollars.

It's abused when companies deliberately fire people before the options vest - this is relatively rare, since any sane company knows that getting rid of all your talent is a rather short term option.

This is what Oracle did when they took over Sun - they fired almost all of the VPs before the deal so they would have no share. In their case it was more justified, these people hadn't contributed to Suns future (it didn't have one) and weren't the reason for the Oracle takeover - there was no reason why they should gain from Oracle being in charge when the music stopped.