Diaz's treatment is almost certainly due to him being a repeat offender, unlike Overeem.
Diaz, a California native, is currently suspended by the Nevada State Athletic Commission for testing positive for elevated marijuana metabolites for the second time. The offense has prevented the UFC from promoting anything that includes Diaz.
Meanwhile, Overeem seems to have put on the proper face in order to get back into the good graces of the commission because despite his suspension, the UFC has been given permission to promote Overeem's next match against Antonio Silva in February at UFC 156.
Overeem seems to be on the right track and no one has made any issue out of his being re-licensed. But Diaz looks to be in a fight he can't win until his suspension is up on February 4. He still has the opportunity to continue to fight the suspension but with it being his second offense, he has a much harder case to prove than Overeem.