Something, someone, or some event that is so unnecessary that it becomes necessary.
It is effectively a way of allowing you to be a total dick, for no reason, and when questioned about your unnecessary outburst you can shrug it off as being negatively-necessary.
Negatively-necessary is in fact negatively-necessary itself.
Marcus 'Wtf Ben why did you draw that dick and label it Marcus, what did I ever do to you?'
Richard 'Yeah dude, that was totally unnecessary.'
Ben 'It was so unnecessary it was negatively-necessary.'
Marcus 'Oh, fairplay.'
Unnecessary censorship has become so unnecessary it is now necessary for entertainment, unnecessary censorship is negatively-necessary.