People usually write their goodbyes so that people who are interested know they weren't banned ... ban deleted or other and no long reside there, as such they will not continue to check that particular e-space. Many goodbyes go on to air grievances / problems that ultimately left them little room to stay. He's just opposed to the direction Twitter is headed.

Why say goodbye before you leave your friend’s house? Just grab your stuff and walk out the door without saying a word! No one will wonder why you left.