It's so they can compress the data and save on your data plan. You can't compress encrypted stuff, so they temporarily decrypt it, compress then reencrypt. The

It's so they can compress the data and save on your data plan. You can't compress encrypted stuff, so they temporarily decrypt it, compress then reencrypt. They could have turned it off for HTTPS data though, but then all the mails would not get compressed and people would have said the browser doesn't work.