Hillary is being forced to turn hers over because she was Secretary Of State and by law she is required to have all emails available for viewing. It is not against the law to have emails on a private server as a public figure, but it is against federal law (Unites States Code or U.S.C.) to hide them. The bottom line is this: No one will likely ever know what was deleted from Hillary's private server. When she initiated use of the Clinton server, she knew she had the capability in her hands to cover her tracks. She failed a key test for good government. But like her husband, both are former attorneys and well aware of the "appearance of impropriety" standard of conduct. Neither Bill nor Hillary have ever been known for having a sense of proprietary behavior.
Anyway, this is no different than you having your own private email server that government cannot get their prying eyes on (read: not a Yahoo or Gmail account that is on those company's servers that the government can easily, and legally, obtain access to thanks to the Patriot Act). If you have your own secure private email server and account, you can tell the government to piss off and get a warrant or otherwise say "oops" and delete the emails after backing them up to portable media backup devices and hiding them anywhere you want to.
"The head of each Federal agency shall make and preserve records containing adequate and proper documentation of the organization, functions, policies, decisions, procedures, and essential transactions of the agency and designed to furnish the information necessary to protect the legal and financial rights of the Government and of persons directly affected by the agency’s activities. (44 U.S. Code § 3101)"