And 18 USC § 2703(c)(2) requires no more than an administrative subpoena (though there are other ways to get it as well). And an administrative subpoena is not a warrant at all. But don't take my word for any of this, read it for yourself:
"(2) A provider of electronic communication service or remote computing service shall disclose to a governmental entity the [long list of customer info skipped] of a subscriber to or customer of such service when the governmental entity uses an administrative subpoena authorized by a Federal or State statute or a Federal or State grand jury or trial subpoena or any means available under paragraph (1)."
Ref: http://www.law.cornell.edu/uscode/text/18/2703
It's like we have a software patch and everyone is reading only the patch, without considering what the existing code does.