I'm sorry you had such a negative experience; that sounds most unpleasant. Unemployment in and of itself isn't a bar (students can get it), so there must have been something about your story or demeanor that they found suspicious. It sounds that they weren't satisfied that you had means to support yourself which puts you into a higher risk category in their eyes. It also sounds like they honestly thought you were being evasive and acted like jerks in return.
ESpen36's analysis leaves out the minor detail that TSA is plugging GE as a way around being groped, so people who don't travel internationally are also applying now.
Due process for something like this can be satisfied by something as simple as an administrative appeal. I suppose a final agency order could be reviewed under the APA.
My son's recent interview for GE supports your statement about the TSA and GE. My son has had no international travel in the last three years and is not employed (a full time grad student).
No issue whatsoever with GE.
The CBP officer performing the interview asked my son what airline he normally flies and had documentation on how to enter the PASSID for all the participating airlines.