They sorta did, actually. Saix said in Twilight Town "Sora, you just keep on fighting those Heartless" and Sora was perfectly alright with it.

The time when Sora realized that it wasn't okay anymore was after the 1000 Heartless battle, when he realized that Organization XIII was taking those hearts which Sora released with the Keyblade and re-directing them to create a giant collection of hearts (Kingdom Hearts), rather than letting the hearts return to their original bodies like they should.

They kidnapped Kairi so that Sora would go berserk and storm The Castle That Never Was in a fit of rage and lure him into their trap.