Here's the breakdown:
* Rowley has been consistently getting into trouble: He's been disruptive in class, causing trouble with Greg, and generally not following the rules.
* His parents decide enough is enough: They decide to punish him by taking away his TV privileges as a consequence of his ongoing misbehavior.
It's important to note that the specific reasons for grounding Rowley from TV aren't explicitly stated in the book. The emphasis is on his overall pattern of poor choices leading to this punishment.