First, there are multiple examples of federal government employees being fired and prosecuted for mis-handling confidential data to private email accounts. The cases most well publicized involve IRS employees and contractors (usually tax info on celebrities).
Secondly let me state that the rules regarding not using private emails for government communications have been in effect for federal employees since Bill Clinton took office. The Federal Records Act has nothing to do with this prohibition which more than one high-level government official has evaded over time. All these high-level employees should have been prosecuted (Rice, Powell, Clinton, etc.) for their use of private emails accounts - it was not very common prior 2012 (despite your assertion).
Great, show me some of these examples of federal employees being prosecuted, enough of talk, let's see some facts.
Second, you are wrong
There's not any blanket prohibition on any federal employee from using a personal email account to conduct government business," said Potomac Law Group partner Neil Koslowe, a former Justice Department special litigation counsel who has worked on cases involving the Federal Records Act.
http://link.nationallawjournal.com/54b6ae9e92721943738b547c2d1ph.byg/VP0oosPoT2Gu8FvZAd768
Third, if indeed there was a law, can you provide the statute so we can prove the National Law Journal wrong. What do they know, maybe they are not peer reviewed either (they are)