You are the one assuming. But let me clarity.
Example:
The DM runs a sandbox game. The players, having heard stories of the Red Wood and it's ancient elven ruins, decide to ignore the warnings that trolls live near the ruins in great numbers. The players figure they can either parlay/bribe the trolls or run away.
As they make their way through the forest, they are surprised (ambushed) by a couple of trolls.
In 3.x: At least two, if not more characters die the first round. If surprised, they might not even have a chance to attempt a non-lethal resolution.
In 4e: Chances are that noone will outright die in the first round. The players are still SOL, but at least the get the chance to run.
This example illustrates what I meant. I was not talking anything about combat always being an option. That's the whole point. It's not. They are still going to die, but in 4e, they will at least live long enough to have a chance to do something else besides fight. Often that was not the case in 3.x. IME anyway.
I consider that a very contrived example or a very naive one, and illustrates that you missed my point about combat not being the only option. You simply assume that the players stumble into the trolls and get into a combat. That's really back to my take on an undesirable situation or poor sandbox DMing: combat is the only option.
You stumble into a hex with a troll in a sandbox game with a good sandbox DM*, the DM will consider what the prevailing situation in the area is. Do the trolls influence the environment around them? Do the PCs find carcasses? Tracks? Remnant of unfortunates who ran across the trolls?
When you are doing setting driven DMing, the monsters in the hex aren't the only thing present in the environment, and the terrain is more than just something to complicate your power uses.
If this really was the players' fault (e.g., as you said, they assume they can parley with trolls and ignore warnings), then that might serve as a harsh lesson. One that rolling up the new characters should help hammer home.**
But trolls surprising players without warning in a sandbox game? That simply wouldn't happen were I to run. Trolls are not stealth hunters; they are used to ruling by brute force.
* - And really, you could write a DMG to educate would be good sandbox DMs.
** - In BD&D, rolling a new character might be pretty quick. In 3.5, I might contrive to let them live with some major consequence, e.g., they wake up naked hanging over the stew.