Jeff Wilder said:
By the rules as I understand them, even if you're holding weapons on someone to threaten him, initiative is rolled when combat starts, and you've blown your chance at surprise.
The players CANNOT affect surprise. Surprise is determined by the DM - is one side of the potential combat aware of the other when the other is not? If players obtain surprise but never consider anything but attack when they have it - that isn't the rules, it's the players. The hard rules of combat do not dictate to the players that they SHOULDN'T ever roleplay an encounter if they start out with a tactical advantage by choosing dumb, brute force instead. The rules only confer the tactical advantage - the players are UTTERLY responsible for how they choose to use it.
Players who never parley at the outset of an encounter are either bad or unwilling roleplayers, probably trained to be so by a DM who never has NPC's parley instead of attack and/or never confers any advantage or useful outcome to the PC's when they try parley.
I realize a lot of DMs house rule this, but I'm interested in RAW. Am I missing something that would encourage my players to pause and talk to the Bad Guys before slaughtering them?
Only you would know. You can lead a horse to water but you can't make him drink - but are you actually leading them to water or a dry well?
Part of it is that you need to give strong indications that talk might be better than mindless violence or the PC's DON'T have a reason to sacrifice tactical advantage. When they DO try to talk first you can't just repeatedly have it degenerate to a fight ANYWAY or you're only teaching them that trying it is a waste of time. If they are already addicted to first-blood tactics then you have a job on your hands training them to merely SOMETIMES take a different approach.
The way to do that is to start throwing encounters at them where there is just NO WAY to handle it by blunt force. The initial choice to try talking has to be rock-obvious (and even then they might keep trying force out of sheer habit). After you have them trained to react appropriately to encounters with a talk-only arrangement then give them a choice of non-combat options. Talk to it or sneak past it. Talk to it or get someone else to fight it. Then add encounters where the opponents will start to fight but quickly and frantically try to parley. And so forth.
Eventually you should be able to get yourself and your players trained to consider the possibility of non-combat options, and to offer clues about/recognize those opportunities when they arise.
But you notice that none of that is a matter of RULES manipulation... It's all roleplaying.