There are all sorts of reasons . . . They may have a set of specialized courses or programs that they want to market to you. They may have scholarships available to certain industries. They may be looking for people to teach or consult on specific programs that they want to offer.
Many schools offer special pricing to active or former military. This school may offer these prices to others working in defense, or other industries and it’s easier to start by asking if you have a clearance, than to try to list the industries that would quality.
Having said all of that, I have been looking at online master’s programs in psychology. Every one asked me if I am active military or a veteran. Not one asked if I have a clearance. It certainly MAY be that you are looking at an organization that you may not want to be affiliated with.
Remember, the Chinese acquired some of the information needed to develop their own Aegis type navy ship by forming a company, wholly owned by the Chinese military, which managed to get contracts to work on the U.S. Aegis program!