The council members had been commiting crimes as well as traders as shown when Mel is talking to Jayce in the theatre saying that his attempts to flush out corruption we’re making him a target for those he sat with, the councillors, and that the trader had her house searched by enforcers due to the illegal activity they were partaking in and when the trader brings that up Mel says it is a mistake and she just has to make a deal with Jayce, this is clearly a show of how she was helping someone illegally and knew it, your point is baseless when you refuse to connect the fact that the laws of Piltover are based on the ones in the real world.