And that's pretty much exactly why he agreed--if they don't tell Scott he'll probably just manage to get in the way with no idea what the hell he's getting into. This way, at least, there's a chance he won't do something stupid. Or, you know, a chance he'll do something incredibly stupid behind their backs. Which is part of the reasons Derek freezes when Stiles goes on.

"Tomorrow night? And what do you mean you're going back? To Scott's house? With the Alphas right next door?" Are you surprised he's not happy about this, Stiles? "How about texting them-" Not calling, because christ knows if the Alphas are listening in at Scott's house. It wouldn't be hard. "And getting them to your house tonight." To give Scott that much less time to go try and cook up some dumbass plan to stab him in the back with Deaton.