He worked for the contractor that services the Shuttles. It was the contractor that fired him. Whether NASA leaned on the contractor to fire him or not isn't certain. If I remember correctly, they claimed he was revealing proprietary information on his website.

Very little that NASA does is really classified. They did have to deal with classified info when they were launching military payloads in the 1980s, but those days are long gone. The last military launch on a Shuttle was over 10 years ago. Most of what NASA does is part of the public record.

In the past couple years, a lot of information that was put out in the open or declassified previously is now being restricted. What with the number of countries trying to develop long range missiles, even old data on reentry vehicles and such could be useful to them. Just because you would never use a 30 year old RV design, it doesn't mean that info wouldn't be useful to North Korea or Iran.

A few years ago, it was common for military websites to have all sorts of information about bases, units, missions, weapons systems, etc. It was a goldmine to people wanting to find out about the military, including those who wanted to do it harm. In security training, they used to discuss EEFI (Essential Elements of Friendly Information). This are unclassified pieces of info that when put together can reveal classified information. The formal name for this is Open Source Intelligence. The classic story is when Tom Clancey was writing his first (published) book, "The Hunt For Red October", he was an insurance agent who read a lot of military trade journals and references. When the Secretary of the Navy read the book, he hit the cieling demanding to know "Who the hell authorized this?" Clancey was invited to speak at the CIA on how he put together some of the more sensitive aspects of the story.