Just a reminder:

ALEX: Locke.
[Both Ben and Locke turn to see Alex]
ALEX: I hear you're gonna go see Jacob. You're gonna need this.
[She holds out a gun for Locke. Ben looks on with a look of surprise]
LOCKE: [Taking the gun] Thanks.
*********************
BEN: We're here.
[Locke shines his flashlight at Ben]
BEN: You'll wanna turn off your flashlight, John.
LOCKE: Why?
BEN: Jacob feels the same way about technology as you do.
[Locke turns the light off and they leave their bags outside. Ben reaches the door and lights a lantern]
BEN: Once I open this door, there's no turning back. You sure this is what you want?
[Locke steps forward and looks at him resolutely]
BEN: So be it. [To the door] Jacob? I'm here with John Locke. We're coming in now.
[Inside Locke looks around the room. Jars stand by the window, a painting of a dog can be seen by the wall. Ben moves over to an empty chair]
BEN: Jacob. This is John. Aren't you going to say hello, John?
LOCKE: What?
BEN: [Looks to the chair] I told you he wouldn't.
LOCKE: Who—what are you talking about?
BEN: You can't see him?
LOCKE: See who?
BEN: Jacob! He's sitting right here, in this chair! [Looks at the chair] Yes, I know, but he insisted.
LOCKE: What is this?
BEN: You wanted the secrets of the Island? Well, here they are. This is the man who can answer every single.... [Stops, pauses while looking at the chair] I am not. He made me bring him here—do you think that that was my—sorry, may I finish?
LOCKE: You're crazy.
BEN: Excuse me?
LOCKE: You don't know anything about the Island, do ya. You just...made it all up.
BEN: [Looks at the chair] Jacob, please, I can't hear him if you're gonna talk over what he's saying—
LOCKE: Stop. Stop. Shut up! You're putting on a show for me? Or do you...do you really think there's someone there?
BEN: I know there's someone there!
LOCKE: You don't know anything.
BEN: I'm sorry you feel that way, John. And I'm sorry that you're too limited to see.
LOCKE: You're pathetic.
[As Locke leaves he hears a voice]
VOICE: Help me.
[Locke turns]
LOCKE: What did you just say?
BEN: I—I didn't say anything.
LOCKE: Oh yes you did. I heard you, [raises the flash-light] you said...
[Suddenly the room takes on a life of its own. Chairs rock, glass smashes, the lantern catches fire and windows shatter]
BEN: [To the chair] That's enough, you've had your fun!
[Ben is thrown back against the wall. For a brief moment, a figure is seen in the chair, followed by a closeup of an eye. Locke runs out of the cabin. Ben walks, shaken, from the cabin a moment later. Pause]
LOCKE: What was that?
BEN: That was Jacob.
****************************
I'm wondering: if Ben could not even see Jacob, nonetheless hear him, why didn't Ben freak out when the cabin started going nuts? Instead, he went back to the chair and started talking to "Jacob", even telling him to stop and that "you've had your fun", like he had experienced this before.