It's important to keep in mind that it takes two to tango. In rare instances, reporters (or foreign intelligence services...) can piece together classified facts from other unclassified data - hence the need for good OPSEC. In almost all cases where you see obviously classified information in the media, however, it is because someone, somewhere, disregarded their oath to protect classified information. In most cases they do so because they believe they are furthering a greater good - the country's, their Service's, their bureaucracy's, etc. In the *vast* majority of cases, it is not done by what are known as "original classification authorities" making downgrading decisions - those are very involved processes. Instead, it is "senior Administration officials" advancing a specific agenda with a given reporter. Or someone in Service A trying to stick it to Service B by leaking something adverse. Or someone currying favor with Reporter C by leaking some interesting tidbit.
What bugs me most about leaks is that most people doing the leaking have no idea about the adverse consequences that can come with public disclosure. To the policy types, they look at the info and say "I don't see what the harm can be from telling a reporter this", or "I don't even see why this is classified." In some cases, they are right - there's no obvious harm. In other cases, millions (to tens of millions, to god knows how much) of $$ can get flushed down the toilet because someone talked about a collection or an operational capability. Worse is when HUMINT gets leaked, and someone gets strapped to a board and stuck in a roaring furnace because someone talked. (Yes, it happened.)
Classified information is like crack to the media - they crave it, so they can get the scoop, and go to great lengths to cultivate sources. Both the Administration and the media knowingly use each other to advance their own agenda. The whole Valerie Plame episode is a great example of how this sort of thing is practiced in Washington, and how it can spin out of control without too much difficulty, resulting in jail time and ruined careers. The takeaway should be a simple one: if you've signed a Non-Disclosure Agreement, then STFU. Remember - reporters are not your friends, and like flight surgeons and security folks, you can generally at best break even when you're talking to them....