I think that's a reasonable position to take when actually designing adventures, but when designing a setting/sandbox, it doesn't hold. I mean, big powerful things are likely to be well known (feared), at least enough to give a clue they are there, but if the 3rd level PCs head into the Swamps of Doom where the hag covey lives (and likes to dine on travellers) they are responsible for their demise, not the DM.
But, do you as a DM, inform the players, either in game or out of game, that there is a hag covey there beforehand? Do you explicitly warn the players that going somewhere is pretty much a death sentence, or, do you let them go forward with their plan, even though they may not know that it's certain death?
Now, if you've dropped the explicit warning and the PC's STILL go there, then fine, toast them.
But, I'm not really even talking about encounters that are so far above the PC's level that it's guaranteed death. I'm talking about how encounters which are reasonably defeatable (say EL +/- 2) that kill PC's.
I think it really depends on what kinds of opponents you use. If you use a lot of classed monsters, particularly classed humanoids, I don't think you'll see this problem as often. Classed humanoids tend to be on the very weak end of their CR, particularly non-caster classed humanoids. Compare the damage potential of a 5th level orc fighter to a troll and you'll see where I'm coming from.
If, OTOH, you're like me and use a lot of monsters, this becomes a much bigger issue. Monsters of a given CR can just do so much damage that a single lucky roll kills a PC. I wouldn't mind if a single lucky roll really hurt a PC. That's fine. The troll nails the fighter three times, rakes and rolls good damage, and the fighter is really wounded, that's cool. The troll nails the fighter three times, rakes and rolls very well and takes the fighter from fresh to dead is not. A troll can do 50 points of damage in a round, without crits. Lots of 5th level fighters don't have 50 hit points. 4th level fighters probably don't and a troll is a reasonable encounter for a 4th level party. Bit tough, but not insanely so.
That's where my problem comes in.