Letting Pence off while attacking Hillary for her vote history would be the same accepting Hillary's mishandling of classified material while jailing Manning or Snowden for their mishandling of classified material.
It wasn't just "mishandling" by Snowden. It was deliberate leaking of information that he knew for certain was classified. Hillary just used a private email server, which has been done by several other politicians, and no information was properly marked as classified when it was sent or received.