The film plays with several themes around trying to make the kids feel like they are crazy. For example When Beverly and her bathroom are covered with blood her dad acts like nothing is wrong. The kids are afraid to share their various sightings of Pennywise with each other for fear of the others thinking they are crazy.
All of the TV shows seems like altered versions of regular types of shows - News Program, Kids show, etc. This suggests that the kids see different images and hear differently than adults do. We see the adults checked out in front of the the tv and not responding when they hear (paraphrased) "Sewers are public property - come play in them - they're for you." The Kids watching tv see the nefarious message that "IT" wants them to hear which feeds their fear and paranoia.
Whereas most of the TV scenes are intended for a generic audience i.e. "Float with us" or "Come to the sewers", etc., the scene with Henry is a specific to his fears and worse instincts. He is afraid of his father who had just shot his gun around his feet. IT not only causes fear but makes people act on that fear in the worst way.
It's hard to believe that the bullying was not similarly heightened because of the malevolent presence of IT. Henry and his gang were known to rough up the Losers, but carving his initials into someone's belly is ratcheting it up a notch (as is killing his own father).
I think the point here is that fear causes violence so the TV messages not only create fear, but also encourage violence.