I think it's more the purpose of the map, Dox.
Idle maps arn't intended to be played, they are intended to provide an environment that allows players to join a server to collect weapon drops without damaging their player stats. Providing anything more makes it "not" an idle map but something else.
When people do "play" an idle map it's essentially a death match (because of the no-victory conditions aspect you already mentioned). So we wonder why people don't just make it a DM gimmick or box map instead.
People make idle maps because it's simple, but most importantly of all quick to finish. Secondly, not all, but most of these people want immediate recognition for their "work". They want to be praised for their efforts as they've seen idle maps spread like the bubonic plague and want their slice of the "market". The "quickness" in finishing such a project is attractive to a prospective author. Understandably we all want to see the wonderful end result of our map asap, but none of the active members here have resorted to producing otherwise intentially broken maps in an effort to do that. Though we may fall victim too impatience on some of our releases.
Orange maps are not that much different. The very implication of an orange map is one that consists of a meat grind that never truely ends. They attempt scenario's otherwise outside of the engines capacity to perform well/properly and outside of TF2's intended gameplay mechanics. Which results in class imbalances, impossible maps and horrible optimisation (despite the basic-ness of the designs). So it's not hard to see why people generally assume these people have no idea what they are doing and need to read a couple design articles (which would have been the first step if they intended to take a mapping project seriously, and would have resulted in not producing an orange/idle map).
If someone posts an orange map and asks for feedback it's only logical then that that feedback consists of telling the author to make it more aesthetically pleasing and balanced (as you say). But with each piece of "valuble" feedback we inevitably take the author more and more away from it being an orange map. Advising a modification of the layout to compliment TF2's gameplay mechanics usually results in the author ignoring you and accusing you of not understanding the concept of the map; which is supposed to be a "different experience" to what is currently available. They want non-standard TF2 gameplay (gameplay that's actually available in other games) in TF2, and we're not supposed to laugh at that notion when they intend to do it with just the execution of a map layout and expect it to work without flaws; or flaws that can be ironed out with feedback that doesn't require the map to conform to current layout trends. Trends which happen to be dictated by the player class designs and engine limitations.
People generally like orange because of its "no strings attatched" gameplay that doesn't require them to complete objectives to "win" but merely see what kind of KD ratio's they can achieve free of the anxiety they might experience if they might "lose" a game. It's practically a requirement for your layout and respawn times to break your game modes playability. When you enter an orange map you're not expected to win or lose, just frag.
If someone wanted valuble feedback on their orange map the first valuble piece of advice i would give would be not to make an orange map. That isn't me, as a serious level designer, trolling their efforts. It's the way the cookie crumbles. Is all they want to know is that it's "fun" to play, and the only mistake they can really make without having compile errors or setting up the gamemode wrong, is whether they made the map too small.
This isn't intended as a rant, although it might look like it because of its length. But what you are asking of people is unrealistic. The moment you start actually providing feedback to an orange or idle map you effectively troll it with expectations.
edit: made post smaller. >.>