Probable cause and a warrant is required when there is a 4th amendment right. The anonymous poster does not have a 4th amendment right over information about his posts in the hands of a third party (in this case, Reason). Reason, of course, does have a 4th amendment right over that information. But a warrant with probable cause is not required for a grand jury subpoena, because the recipient has the opportunity to challenge the contents: http://jolt.law.harvard.edu/articles/pdf/v24/24HarvJLTech543...