Speaking only as an average citizen:

It's impossible for law enforcement to enforce all the laws all the way all the time. Without the discretion to give warnings instead of citations or arrests, and to frankly ignore certain common minor infractions, the job of law enforcement would be literally impossible. So discretion is necessary.

The problem is that "necessary discretion" can too easily become a cover for biased enforcement, or hassling people for personal reasons, or distorting the rules just to make the job easier for the enforcers, or even for corruption. This is basically a management problem: law enforcement management needs to have clear guidelines on how to manage discretion, and to enforce them.

The TSA, from the reports I've read here, seems to be doing a fairly poor job in managing discretion. For instance, on flyertalk there have been claims by people, some of them claiming to be TSA staff, that you can't bring medical substances through if in the opinion of the screening agent you have "too much" of it, or that if you forget to put a bottle in the baggie you must be punished by being sent to the back of the line. So far as I know, these are significant alterations of the rules which are just made up by TSA staff on the fly and aren't documented anywhere so that people can be aware of them. The making up of such rules, and the keeping of the public in ignorance of them until someone breaks them, is an abuse of discretion.