That is a presumption of 3e I believe, one that I have grown to have some level of distaste for. IMHO, the world should be the world. It does not alter and shape itself according to the abilities of the PCs who adventure within it.
You believe incorrectly. RTM, get back to us.
We need to change the system to take the "Mother May I" power away from the DM (most of them are incompetent, anyway -- you're lucky to get a good one)! The system should enforce a certain percentage of fighting encounters as a rule.
A good game will be able to turn a poor DM into a relatively good one. 3e is definately moving in this direction, and part of how they're doing it is by "assuming a baseline." Which may very well include a certain percentage of fighting encounters as a guideline. Certainly most people need advice on how to run a good game, and suggesting how many fighting encounters a given night of gaming can have would be a good way to start determining what's good for your group.
Oh, and I hate anything too dangerous or "bummer inducing" (level loss, save or die, rust monsters chomping on my vorpal sword, et cetera). Bummer inducing stuff has got to go, too. Bummerism are not fun...
If I'm going to spend 4 of my precious hours in a week doing something, it had better not bum me out, because if it does, I won't do it for long. Mean things suck.
The concept of "balance" in D&D is a lot of a baseline. No one I know plays a "baseline game." No one I know plays with four characters (a fighter, a cleric, a wizard, and a rogue) using rules only from the Core, generating dungeons with the rules in the DMG and putting those dungeons near towns for refuelling stations and using minis on a map.
But the existence of a baseline is useful because it allows one to measure various qualities aobut your game and adjust them to a level you and your frineds will find fun. That baseline, then, should be balanced, it should provide options of roughly equal player benefit, so that when you're changing things about the baseline you can then trace the source of discomfort to it....if your party lacks a healer, maybe that's why there's a high mortality rate. If your party seems unable to handle the encounters, maybe it's because they don't have enough treasure.