This was later changed, as the Xbox LIVE Arcade and PlayStation Network re-release of the game features Metal Sonic, Dr. Eggman in his mecha-armor, and Honey the Cat as officially playable characters without the need of hacking. However, the other dummied out characters like Rocket Metal Sonic unfortunately remain unplayable.
This game is the second of two Classic Sonic appearances that Espio the Chameleon makes. His only other appearance was in Knuckles Chaotix. Curiously, the other Chaotix members are completely absent.
The Japanese manual for Chaotix makes it clear that all the characters had their own individual agenda for going to the island, and thus not really a team. In light of that, it makes more sense that they'd act of their own accord afterwards.