Why did the feds grant immunity to Hillary's "highly improper" aide?

Comey knew it was Mills who had Hillary’s e-mails moved off her private unsecured server and onto laptops, where she decided which ones were government-related and OK for public release and which were “personal.” He knew it was Mills who shredded the e-mails that were printed out and who had the rest of the 31,000 e-mails deleted, and then had the laptops bleached clean.

And he knew it was Mills who told the Denver tech who maintained the server to stop retaining her e-mails and to delete Hillary’s archived e-mails, all of which the tech dutifully performed after Congress subpoenaed them and ordered them preserved.

Even so, Comey agreed to grant Mills immunity in exchange for her cooperation in the investigation. He also agreed to ground rules that left some lines of inquiry off-limits. When agents in April tried to pin her down on the procedures she used to search for Hillary’s e-mails under order, she and her lawyer stormed out of the room. So much for Comey’s cooperative witness.

Mills claimed such information was protected under “attorney-client privilege,” which is ridiculous. Mills was chief of staff for Hillary, not her lawyer, at the time Hillary was bypassing government security and squirreling away state secrets in her basement.