What a tough question, Chuck!
As a lifelong Catholic, I find this a tricky area. Of course, I'm not a bishop, so it's way, way above my pay grade.
First, it's important to realize that Catholics believe the bread and wine we consume are actually the Body and Blood of Christ. It's not supposed to be symbolic. It's the real incarnation. If Catholics truly understood what was happening at Mass, they would, I think be much, much more reverent.
We invite Christ himself into us in the Eucharist; we are supposed to be clean of sin when it happens. Here's the problem. Promoting abortion, for example, is a grave sin. How can Bishops allow these "catholics" to receive?
I did some research.
Cannon Law 915:
Those upon whom the penalty of excommunication or interdict has been imposed or declared, and others who obstinately persist in manifest grave sin, are not to be admitted to holy communion.
The trick, as I understand it, is how to decide if a sin is "manifest," and are the offenders "obstinate." Those definitions are broad.
Some bishops say it is the reponsibility of the Communicant to make sure he or she is properly disposed.
Many traditional Catholics feel this rule has not been applied clearly enough, and because of that, priests and bishops are now afraid to follow it.
Does this help at all?