The contents of his memo are immaterial.     Eg:   He might have had one line in his memo and still be fired (eg:  a link to a hardcore site).      His job was as a Software Engineer.    This memo had nothing to do with his job, thus he opened himself up to being fired when he hit the "send" key.

I can't believe the number of people who believe that an employee has a right to spend time and company resources doing anything they want with no consequences.    Every action has consequences and if you can't accept this, then it's best not to take the action.     Otherwise, live with the consequences with no whining allowed.