As you probably already know, a "rule" isn't the same a a "law". Even breaking a law doesn't usually lead to criminal prosecution because most laws are civil, not criminal. And, most salient, the relevant standard for criminal prosecution w/r/t classified information require intent. HRC obviously did not have criminal intent. Moreover, nothing actually happened: while the setup may have been risky, there's no indication it actually had any consequences (beyond those for her career).

You can contrast this with the Republican attitude to Gen Patreaus, who intentionally gave classified information to his girlfriend and who was vigorously defended by the political right.