Why not Knuckles versus Sonic?

I recently noticed an unfair situation. On S&K there is a stage called Hidden Palace Zone. On Sonic's path, you face Knuckles, who later realizes Robotnik is the true villain of the story. But on Knuckles' path, when you arrive on Hidden Palace Zone, You just go to the portal to stop at Heavenly Sanctuary! I mean: If Sonic can fight Knuckles, why can't Knuckles fight Sonic?

This is Sparta!
-- King Leonidas