Lyxen
Great Old One
Climbing a wall takes time, and others thus have time to stop the climber if they want to. Throwing a dagger is immediate, and the odds of someone being able to stop that action are slim at best.
Not necessarily, climbing the wall could be just jumping and scambling up a 3 foot wall to escape an enraged beast. For me, the speed of the action is barely relevant (unless it takes a really long time not relevant to combat).
A wall can be difficult enough to climb that a DM is within her rights to declare it's an auto-fail. Throwing a dagger invokes a different set of rules - to wit, combat - wherein a DM does not have the right to declare an auto-fail without a roll but must instead at the very least allow a roll to hit...quite possibly against a surprised target, if the action was unexpected.
But there is a roll, it's called initiative, and it's called whenever someone declares an intention that triggers the fight, not when that action resolves.
And to the point of the thread: if the throwing of the dagger is the trigger that starts the combat and thus off which everything else is timed, the pouncing killer simply shouldn't be able to cover the distance before the dagger is away.
Which is why the trigger is NOT the throwing of the dagger, but the declaration of intention that a dagger should be thrown. And for me that's the right way of conducting things.
NPCs get the same break: the PCs would get a surprise roll to determine if they saw the dagger coming (in my game if you're surprised you don't have your active defenses up, which can make a big difference to your AC) but the dagger would still be thrown before anything else could happen.
Of course, it's symmetrical, but see above, my trigger is earlier than yours.
In the case of a player declaring an unexpected and sudden action such as whipping out a dagger and throwing it in an otherwise peaceful situation, I don't see a valid reason to not allow that action to happen and resolve.
You see no reason in your paradigm of the game, but this is not the way the game is described, and the way it's described is absolutely right for me, considering the genre that I've read/watched and my LARP approach to this. It's just a matter of preferences.
That being said, again, there are cases in which I would make a local ruling like yours, in particular if the action starting the fight has no reason to be observed by anyone. But then, the surprise rules (with a local ruling) would take care of that for me.