The rules usually dont apply to top level folk.... he could have come clean, admitted the affair, patched things up with his wife and kept his job. Noone in either party wanted him out.

I personally suspect that this is a prelude to his own Presidential ambitions, 4 years is plenty of time to recover from a minor mistake that never blew up into a scandal. His timing of waiting until after election ensures that noone accuses him of partisanship.

For who could be free when every other man's humour might domineer over him? - John Locke (2nd Treatise, sect 57)