"It's not that they don't have any other heat source in their home. Most have electric baseboard or electric radiant in the ceiling. They just choose not to use it."
Think about what you have just said.
If they have electric or other, and they turn that system off in lieu of a vent free fireplace, the vent free fireplace is now their Primary Heat Source, not a Supplemental Heat Source.
Therein is the problem.
See my earlier post on this thread regarding problems that I have personally observed with VF gas fireplaces. I was not exaggerating about the soot problems.