While it's reasonable that Pincus wants to find some way to obtain more shares other than buying them, sending an email like this could not be beneficial to morale.
If it is truly in the best interest of Pincus and Zynga to obtain the shares, then it's also in the best interest of the targeted employees to give up the shares in exchange for something else with equal or greater expected value.
Since the hubbub around this reflects poorly on the firm as a whole, the employees refusing to cooperate are likely shooting themselves in the foot, though it also seems that Pincus has blundered significantly.
Pincus at least seems to be acting rationally about the future of Zynga, while the employees (even if they have reason to be annoyed by this) come off as being motivated purely by greed, spite, and short-term considerations.