I'm thinking it's a combination of things.
Part of it is probably the lack of fire making/cooking skills, or the lack of interest in a campfire for time/effort reasons.
I also think "some" view elaborate "fireplaces" as not keeping with the no trace idea or maybe too elaborate, like the sites that have "camp furniture" and cooking areas made from rocks and dead logs.
I've seen fire pits that were torn apart down to a couple of rings like you describe. I might not take the effort to build a real functional "fireplace", but I wouldn't tear down anyone's effort either.
 I cook on stove, except for grilling fish. Having a nightly campfire, especially in early spring, is just a must have for me. Even if its real small and smoky, something about tending a fire, just makes the end of a good day, even better.
 Man has been building fireplaces and "camp furniture" for eons. I don't have a problem with Native materials being used, as long as it doesn't include cutting green timber.  Here's an "Elaborate Fireplace" from Quetico Lk.
 I'm much more offended by the garbage left in camp/fireplace, especially the TP and Human waste left in the camp area.
 I just can't wrap my mind around how ANYONE would think that was okay too do >:(
 Are these people THAT big of pigs, that they go in their own campsite, or are they people who stop at a empty site and use it as a bathroom?
 I bet some of these people are the ones that are offended by an elaborate/functional fireplace, thinking it ruins the "wild factor" for them.
 Here's a real nice fireplace on Quetico Lk!  Tour guide looks familar too ;D     (You need to Login or Register   (You need to Login or Register