A good comedy rule of thumb is that passive aggressive behavior is usually funnier than confrontation. Admittedly, Garfield frequently disproves or at least flaunts its disregard for this rule. Today we see it fully embraced, however. At first a normal person would wonder why a man would avoid direct confrontation with his own cat. The stakes are low. The offense of having eaten the last cookie is petty, and probably not even punishable. There is no mystery about the offender: Jon knows perfectly well that Garfield is guilty.
The lengths Jon goes to in forcing Garfield's confession may not seem as excessive as Hamlet Act 3, Scene 2, but relative to the crime, maybe they are. And Garfield reacts with glaring rage, but doesn't apologize. What else could Jon have expected or wanted? And yes, that last poster likely says "REWARD," and Jon is taping it up in his own home, as if anyone were there to see it and collect such a reward besides Garfield or possibly Liz. Well, sometimes our desire for small, meaningless vindication is enough motivation.
Q: It is impossible to tell in the context of Garfield artwork, but has Jon done an artist's rendering of the lost cookie? ... Or had he for some reason taken a photograph of the cookie before its disappearance, in anticipation of just this situation? One hopes for the latter, as it indicates a sad acceptance by Jon of Garfield's ability to shape his life.