Gross negligence and extreme carelessness are not the same. Gross negligence is a conscious and voluntary disregard of the need to use reasonable care. Clinton clearly understood the need for reasonable care as she took some security measures and asked people not to email her confidential materials. She merely did a poor job executing on the knowledge that care was needed, which in this case is not actually illegal (especially when grave harms did not occur, which is an additional legal factor).

The laws are written around consent because the goal of the law is to enable punishment of people who intend to harm the US government. Note that Comey said that it's the individual's responsibility to know when material is confidential, even if that is not marked. Yet even information that is widely reported on in the media can still be considered confidential. The law is written to protect those who take "careless" or "negligent" actions in light of the difficulty of the task of maintaining confidentiality during the kind of extensive communications required to execute the business of government.