While I think this is petty and dumb, it’s not really as big of a deal as a lot are wringing their hands over. It’s not like Brennan has a SIPR terminal and a scif in his basement. It’s quite likely he hasn’t interacted with ANY classified material since he left office.
What's amusing to me is the media response. They're supposed to be these experts and explaining why this is this grave crime and everything. But from what I understand, normally former officials and any other workers don't keep their security clearances and can just wander in and rummage through the files or pick up the hottest tips and cheat codes off the 1-900 CIA tipline. If they're needed back to consult or testify or whatever, the agencies can give them temporary grants even as short as a few hours to review material to a month or more if it's a larger case, etc. They can even totally waive the requirements, former Presidents commonly receive that treatment even though they never have been vetted for security clearances and received theirs simply for being President. They even have the power to grant restricted clearances so that say Brennan is only allowed to view stuff from during his term.
Them keeping their clearances was what's out of the norm and likely due to Trump Administration incompetency or something. Obama didn't to my knowledge sign anything to extend their clearances, so the onus must have been on the incoming Trump Administration to approve paperwork of some kind.
Just from a casual glance it seems more like a "bad optics" thing than anything deliberate as either side are painting it. The idea that they are using their clearances to get leaks that they give to their news networks is just as absurd, they're still LEAKING it, which is illegal, even if they're allowed to receive the information under their lingering clearance that just shifts the crime to
themselves rather than the random CIA or NSA employee.