Humiliation Conga: Not only did Pie-Man give Comic Book Guy a well-deserved pie to the face, but it also made him a laughingstock and blew his chance of having tea (and possibly scoring a date with) Nichelle Nichols.
The Lopsided Arm of the Law: As Pie-Man's exploits increase, the Springfield Police Department feels undermined (even if it's somewhat nonsensical, because Pie-Man is targeting Jerkasses, not crime), and so goes after the vigilante with spotlights, chopper support, multiple men in full SWAT gear, snipers... and a "shoot on sight" order.
Noodle Incident: Homer did assault someone before and is warned by Chief Wiggum that he'll go to jail if he tries it again.
Papa Wolf: Homer (as Pie-Man) makes the Rich Texan pay for humiliating Lisa by throwing a pie in his face.
Shrouded in Myth: When Homer exposes himself as Pie-Man, all of the other Springfieldians go on to argue that they had directly seen or heard about Pie-Man, saying wildly diverging descriptions (and none of which fit Homer). Lisa simply says that Pie-Man has ascended to become a "symbol", and Homer exposing himself can't kill that.