Why is it still legal for the government to access and read your email without a warrant?

It is quite likely that many if not most people are under the false impression that their email is private and secure.  Yet, nothing could be further from the truth.  In many ways, an email is akin to a postcard.  While it is in transit from the sender to the addressee, it can be read by anyone who sees it or otherwise intercepts it along the way.  Numerous government agencies, including law enforcement and even the IRS, claim that they do not need a warrant if they want to comb through your emails.  Some lawmakers are slowly beginning to recognize that this represents a threat to the Fourth Amendment rights against illegal search and seizure.  From Techdirt:

Today, in a markup for reform of the Electronic Communications Privacy Act in the Senate, the Senate Judiciary Committee very quickly (like 10 minutes after it started) approved an amendment offered by Senators Patrick Leahy and Mike Lee, which would amend the law to make it so that law enforcement needs to get a warrant if it's accessing your email.

However, the Orwellians among us need not fear.  The Justice Department is already working to help internet service providers to evade illegal wiretapping laws.  From The Verge:

 Internal government documents obtained by the Electronic Privacy Information Center have revealed that the US Department of Justice is secretly helping AT&T and other service providers evade wiretapping laws so that the US government can conduct surveillance on parts of their networks. The legal immunity comes from authorizations granted by the Justice Department through special "2511" letters that absolve carriers in the event that the surveillance is found to run afoul of federal law. . . .

It won't be the first time that AT&T cooperated so directly with law enforcement. It was given retroactive immunity for its role in NSA surveillance programs under the FISA Amendments Act of 2008. That law was passed two years after AT&T technician Mark Klein revealed evidence that the telecom had cooperated with the NSA, installing routing equipment inside a secret room at a network hub in San Francisco.