How is the simplest way to describe closed communion? When you commune at a Lutheran altar, we expect you to only commune at Lutheran altars until you die. That is what we ask of our confirmands - will you face all sorts of wickedness, even death, than fall away from the faith as you learned it?
This also explains the old "deathbed" exception. If someone is dying - well, yes, then this will be the last altar they commune at.
If little Sally comes home and wants to commune but next week she's going to go right back to her mega-church de jour -- sorry. No where else until you die - that's the standard.