Here is a more heavily commented version of Paul’s code to detect and fix the issue (that is also easier to copy and paste).  Please read Paul’s post for more details and background.

-- Fixing a Bug: DBCC CHECKDB data purity checks are skipped for master and model
-- By: Paul Randal
-- Posted on: August 29, 2013 1:15 pm 
-- http://www.sqlskills.com/blogs/paul/bug-dbcc-checkdb-data-purity-checks-are-skipped-for-master-and-model/

-- Turn on a session-level trace flag
DBCC TRACEON (3604); 

-- Look for the value of dbi_dbccFlags = 0. We want it to be dbi_dbccFlags = 2
DBCC DBINFO (N'master'); 
GO 
DBCC DBINFO (N'model'); 
GO 


-- This will set the value for dbi_dbccFlags correctly as part of running DBCC CHECKDB
DBCC CHECKDB (N'master') WITH DATA_PURITY, NO_INFOMSGS; 
GO
DBCC CHECKDB (N'model') WITH DATA_PURITY, NO_INFOMSGS; 
GO

-- Look for the value of dbi_dbccFlags = 2
DBCC DBINFO (N'master'); 
GO 
DBCC DBINFO (N'model'); 
GO 

-- Turn off the trace flag (even though it is just a session trace flag)
DBCC TRACEOFF (3604);