There is a difference in how the laws are written. The speed limit law simply says you can't go over the posted speed. Whether you intended to or not is beside the point. The espionage act, on the other hand, specifically says in the text of the law that you may not intentionally disseminate classified information to anybody not cleared to see it.
FYI, the other half of the relevant law states that you can't negligibly allow classified information to fall into the wrong hands. But the FBI's investigation found no evidence that anybody nefarious did get a hold of classified emails stored on the server, and she did not put classified emails on the server with the intent to disseminate them to anybody who shouldn't see them. Therefore, neither half of the relevant law was broken.
That being said, if it does come to light that somebody did hack her private server and steal classified emails, then it would be a whole different story. But as it is, with the information we know, simply the act of having emails on a private server did not in of itself break the law.