Under U.S. federal law, blackmail is a crime. We know that former US Congress member Dennis Hastert was being blackmailed by someone, and we also know he commited the crime he was being blackmailed for. That person is actually suing Hastert for full payment of the blackmail money. How can this be legal?
While what happened to the victim is horrible, blackmail itself is horrible, and shouldn't be tolerated in a free society. Why hasn't this been investigated?
It has been investigated, it simply has not been prosecuted.
The investigation started when someone reported suspicious activity in Hastert's accounts. The investigation proceeded from a belief that he was being blackmailed. After listening to a wire of a conversation between Hastert and the alleged blackmailer, the officers investigating decided it was not a simple case of him being blackmailed--they or the prosecutors' office used their discretion to go after Hastert for illegally structuring his payments to avoid anti-money-laundering reporting laws, and to my knowledge have not yet pursued any blackmail charge.
Police and Prosecutors have very wide discretion regarding what charges they bring.
In addition, it is very common to have a civil lawsuit brought that implicates criminal laws, but not have the criminal violations be charged. For example, if you steal a purse you go to jail, but if you steal a building you are rarely charged with a criminal offense. It is rarely to a civil litigant's advantage to bring up criminal matters and there are ethical rules limiting the interaction between the two.
One reason charges may not be brought in this case is because prosecutors are often reluctant to file charges if they feel there is little chance of conviction. A man who was abused as a child by someone who later became a powerful politician is likely to be an extremely sympathetic defendant, while Hastert is not a very sympathetic victim. It can be difficult to get a jury to convict under such circumstances, partly because if the victim (that is, Hastert, the victim of the blackmail) has to testify, he may be subjected to grueling cross-examination which explores his history of deception and thus casts doubt on his truthfulness.
Your question reminded me of a story from a few years ago in which a man was charged with beating up a priest who he said had molested him many years ago. Although the defendant admitted beating the priest, the jury acquitted him of felony assault and elder abuse. The jury deadlocked on a charge of misdemeanor assault, but the district attorney declined to retry that charge, saying, "We believe it is unlikely that a new jury would render a substantially different decision."
What evidence is there that blackmail occurred?
From the little I know of the case the facts appear to be that a person (Mr X) was sexually abused by Hastert (and he has admitted that this occurred).
If Mr X had approached Hastert seeking compensation for the civil wrong (as is their right) and the obvious consequence of failing to reach a settlement is that the wrong would be exposed in court as part of a civil suit then this is not blackmail. However, in some states, failing to report a crime is itself illegal.
However, if a third party had threatened to expose it, this would be blackmail.
