I'm normally against government snooping (see my other post), but this wasn't exactly the FBI just being nosey.
Petraeus' mistress was emailing anonymous threats to another woman from the secret Gmail account that the mistress and Petraeus used to communicate with each other. (They wrote drafts and saved them, rather than sending the messages.) That was a big mistake. When the FBI went to investigate the threats (some of which mentioned Petraeus), they had probable cause to get access to the account.
In this case, it appears to have been legit police work that just happened to unravel a much bigger ball of string than they were expecting.
In summary: Women.
